Feature testing assertions documentation (#6332)

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+## Assertions
+
+To help developers write tests the Deno standard library comes with a built in
+[assertions module](https://deno.land/std/testing/asserts.ts) which can be
+imported from `https://deno.land/std/testing/asserts.ts`.
+
+```js
+import { assert } from "https://deno.land/std/testing/asserts.ts";
+
+Deno.test("Hello Test", () => {
+  assert("Hello");
+});
+```
+
+The assertions module provides nine assertions:
+
+- `assert(expr: unknown, msg = ""): asserts expr`
+- `assertEquals(actual: unknown, expected: unknown, msg?: string): void`
+- `assertNotEquals(actual: unknown, expected: unknown, msg?: string): void`
+- `assertStrictEquals(actual: unknown, expected: unknown, msg?: string): void`
+- `assertStringContains(actual: string, expected: string, msg?: string): void`
+- `assertArrayContains(actual: unknown[], expected: unknown[], msg?: string): void`
+- `assertMatch(actual: string, expected: RegExp, msg?: string): void`
+- `assertThrows(fn: () => void, ErrorClass?: Constructor, msgIncludes = "", msg?: string): Error`
+- `assertThrowsAsync(fn: () => Promise<void>, ErrorClass?: Constructor, msgIncludes = "", msg?: string): Promise<Error>`
+
+### Assert
+
+The assert method is a simple 'truthy' assertion and can be used to assert any
+value which can be inferred as true.
+
+```js
+Deno.test("Test Assert", () => {
+  assert(1);
+  assert("Hello");
+  assert(true);
+});
+```
+
+### Equality
+
+There are three equality assertions available, `assertEquals()`,
+`assertNotEquals()` and `assertStrictEquals()`.
+
+The `assertEquals()` and `assertNotEquals()` methods provide a general equality
+check and are capable of asserting equality between primitive types and objects.
+
+```js
+Deno.test("Test Assert Equals", () => {
+  assertEquals(1, 1);
+  assertEquals("Hello", "Hello");
+  assertEquals(true, true);
+  assertEquals(undefined, undefined);
+  assertEquals(null, null);
+  assertEquals(new Date(), new Date());
+  assertEquals(new RegExp("abc"), new RegExp("abc"));
+
+  class Foo {}
+  const foo1 = new Foo();
+  const foo2 = new Foo();
+
+  assertEquals(foo1, foo2);
+});
+
+Deno.test("Test Assert Not Equals", () => {
+  assertNotEquals(1, 2);
+  assertNotEquals("Hello", "World");
+  assertNotEquals(true, false);
+  assertNotEquals(undefined, "");
+  assertNotEquals(new Date(), Date.now());
+  assertNotEquals(new RegExp("abc"), new RegExp("def"));
+});
+```
+
+By contrast `assertStrictEquals()` provides a simpler, stricter equality check
+based on the `===` operator. As a result it will not assert two instances of
+identical objects as they won't be referentially the same.
+
+```js
+Deno.test("Test Assert Strict Equals", () => {
+  assertStrictEquals(1, 1);
+  assertStrictEquals("Hello", "Hello");
+  assertStrictEquals(true, true);
+  assertStrictEquals(undefined, undefined);
+});
+```
+
+The `assertStrictEquals()` assertion is best used when you wish to make a
+precise check against two primitive types.
+
+### Contains
+
+There are two methods available to assert a value contains a value,
+`assertStringContains()` and `assertArrayContains()`.
+
+The `assertStringContains()` assertion does a simple includes check on a string
+to see if it contains the expected string.
+
+```js
+Deno.test("Test Assert String Contains", () => {
+  assertStringContains("Hello World", "Hello");
+});
+```
+
+The `assertArrayContains()` assertion is slightly more advanced and can find
+both a value within an array and an array of values within an array.
+
+```js
+Deno.test("Test Assert Array Contains", () => {
+  assertArrayContains([1, 2, 3], [1]);
+  assertArrayContains([1, 2, 3], [1, 2]);
+  assertArrayContains(Array.from("Hello World"), Array.from("Hello"));
+});
+```
+
+### Regex
+
+You can assert regular expressions via the `assertMatch()` assertion.
+
+```js
+Deno.test("Test Assert Match", () => {
+  assertMatch("abcdefghi", new RegExp("def"));
+
+  const basicUrl = new RegExp("^https?://[a-z.]+.com$");
+  assertMatch("https://www.google.com", basicUrl);
+  assertMatch("http://facebook.com", basicUrl);
+});
+```
+
+### Throws
+
+There are two ways to assert whether something throws an error in Deno,
+`assertThrows()` and `assertAsyncThrows()`. Both assertions allow you to check
+an
+[Error](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error)
+has been thrown, the type of error thrown and what the message was.
+
+The difference between the two assertions is `assertThrows()` accepts a standard
+function and `assertAsyncThrows()` accepts a function which returns a
+[Promise](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise).
+
+The `assertThrows()` assertion will check an error has been thrown, and
+optionally will check the thrown error is of the correct type, and assert the
+error message is as expected.
+
+```js
+Deno.test("Test Assert Throws", () => {
+  assertThrows(
+    () => {
+      throw new Error("Panic!");
+    },
+    Error,
+    "Panic!"
+  );
+});
+```
+
+The `assertAsyncThrows()` assertion is a little more complicated, mainly because
+it deals with Promises. But basically it will catch thrown errors or rejections
+in Promises. You can also optionally check for the error type and error message.
+
+```js
+Deno.test("Test Assert Throws Async", () => {
+  assertThrowsAsync(
+    () => {
+      return new Promise(() => {
+        throw new Error("Panic! Threw Error");
+      });
+    },
+    Error,
+    "Panic! Threw Error"
+  );
+
+  assertThrowsAsync(
+    () => {
+      return Promise.reject(new Error("Panic! Reject Error"));
+    },
+    Error,
+    "Panic! Reject Error"
+  );
+});
+```
+
+### Custom Messages
+
+Each of Deno's built in assertions allow you to overwrite the standard CLI error
+message if you wish. For instance this example will output "Values Don't Match!"
+rather than the standard CLI error message.
+
+```js
+Deno.test("Test Assert Equal Fail Custom Message", () => {
+  assertEquals(1, 2, "Values Don't Match!");
+});
+```