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, 2 insertions, 0 deletions

diff --git a/tests/util/server/src/macros.rs b/tests/util/server/src/macros.rs
index 530d8cea0..542214d20 100644
--- a/tests/util/server/src/macros.rs
+++ b/tests/util/server/src/macros.rs
@@ -30,7 +30,9 @@ macro_rules! timeout {
       let timeout = *timeout.get(0).unwrap_or(&300);
       ::std::thread::spawn(move || {
         if rx.recv_timeout(::std::time::Duration::from_secs(timeout)) == Err(::std::sync::mpsc::RecvTimeoutError::Timeout) {
+          use std::io::Write;
           eprintln!("Test {function} timed out after {timeout} seconds, aborting");
+          _ = std::io::stderr().flush();
           ::std::process::exit(1);
         }
       });