fix(cli): ensure that pre- and post-test output is flushed at the appropriate times (#22611)

Some `deno_std` tests were failing to print output that was resolved
after the last test finished. In addition, output printed before tests
began would sometimes appear above the "running X tests ..." line, and
sometimes below it depending on timing.

We now guarantee that all output is flushed before and after tests run,
making the output consistent.

Pre-test and post-test output are captured in `------ pre-test output
------` and `------ post-test output ------` blocks to differentiate
them from the regular output blocks.

Here's an example of a test (that is much noisier than normal, but an
example of what the output will look like):

```
Check ./load_unload.ts
------- pre-test output -------
load
----- output end -----
running 1 test from ./load_unload.ts
test ...
------- output -------
test
----- output end -----
test ... ok ([WILDCARD])
------- post-test output -------
unload
----- output end -----
```

2 files changed, 15 insertions, 2 deletions

diff --git a/tests/testdata/test/load_unload.out b/tests/testdata/test/load_unload.out
index aef7531af..75187fa7b 100644
--- a/tests/testdata/test/load_unload.out
+++ b/tests/testdata/test/load_unload.out
@@ -1,6 +1,16 @@
-Check [WILDCARD]/test/load_unload.ts
-running 1 test from ./test/load_unload.ts
+Check [WILDCARD]/load_unload.ts
+------- pre-test output -------
+load
+----- pre-test output end -----
+running 1 test from [WILDCARD]/load_unload.ts
+test ...
+------- output -------
+test
+----- output end -----
 test ... ok ([WILDCARD])
+------- post-test output -------
+unload
+----- post-test output end -----
 
 ok | 1 passed | 0 failed ([WILDCARD])
 
diff --git a/tests/testdata/test/load_unload.ts b/tests/testdata/test/load_unload.ts
index 2bd04a676..5027b949a 100644
--- a/tests/testdata/test/load_unload.ts
+++ b/tests/testdata/test/load_unload.ts
@@ -4,6 +4,7 @@ addEventListener("load", () => {
     throw new Error("Interval is already set");
   }
 
+  console.log("load");
   interval = setInterval(() => {}, 0);
 });
 
@@ -12,10 +13,12 @@ addEventListener("unload", () => {
     throw new Error("Interval was not set");
   }
 
+  console.log("unload");
   clearInterval(interval);
 });
 
 Deno.test("test", () => {
+  console.log("test");
   if (!interval) {
     throw new Error("Interval was not set");
   }