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 -----
```

1 files changed, 1 insertions, 0 deletions

diff --git a/cli/tools/test/reporters/mod.rs b/cli/tools/test/reporters/mod.rs
index 6eaed24f1..a152029c4 100644
--- a/cli/tools/test/reporters/mod.rs
+++ b/cli/tools/test/reporters/mod.rs
@@ -49,6 +49,7 @@ pub trait TestReporter {
     tests: &IndexMap<usize, TestDescription>,
     test_steps: &IndexMap<usize, TestStepDescription>,
   );
+  fn report_completed(&mut self);
   fn flush_report(
     &mut self,
     elapsed: &Duration,