feat(test): Add support for regex in filter flag (#6343)

Currently, the documentation makes it sound like the test subcommand's filter
flag could accept some kind of pattern matching value like a glob or a regex,
although the function "createFilterFn" accepts a regex as an argument, there's
no way to pass an actual regex value from the CLI.

This commit makes it possible to pass a string that could be cast as regex 
when string matches "^/.*/$".

With this change, a user can use the filter flag as follow:

deno test --filter "/test-.+/"

Also tested that `\` get escaped properly, on MacOS at least, and this is 
also a valid flag:

deno test --filter "/test-\d+/"

1 files changed, 24 insertions, 1 deletions

diff --git a/docs/testing.md b/docs/testing.md
index f44483125..174123cce 100644
--- a/docs/testing.md
+++ b/docs/testing.md
@@ -129,10 +129,33 @@ There are a number of options to filter the tests you are running.
 Tests can be run individually or in groups using the command line `--filter`
 option.
 
+The filter flags accept a string or a pattern as value.
+
+Assuming the following tests:
+
+```ts
+Deno.test({ name: "my-test", fn: myTest });
+Deno.test({ name: "test-1", fn: test1 });
+Deno.test({ name: "test2", fn: test2 });
+```
+
+This command will run all of these tests because they all contain the word
+"test".
+
 ```shell
-deno test --filter "hello world" tests/
+deno test --filter "test" tests/
 ```
 
+On the flip side, the following command uses a pattern and will run the second
+and third tests.
+
+```shell
+deno test --filter "/test-*\d/" tests/
+```
+
+_To let Deno know that you want to use a pattern, wrap your filter with
+forward-slashes like the JavaScript syntactic sugar for a REGEX._
+
 This command will run any test which contains the string "hello world" in its
 test name for tests found within files in the `tests/` directory.