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, 5 insertions, 1 deletions

diff --git a/tests/testdata/run/websocket_server_idletimeout.ts b/tests/testdata/run/websocket_server_idletimeout.ts
index 85b031b8e..82a35605a 100644
--- a/tests/testdata/run/websocket_server_idletimeout.ts
+++ b/tests/testdata/run/websocket_server_idletimeout.ts
@@ -11,10 +11,12 @@ const { response, socket } = Deno.upgradeWebSocket(request, {
   idleTimeout: 1,
 });
 socket.onerror = (e) => {
+  console.log(e);
   assertEquals((e as ErrorEvent).message, "No response from ping frame.");
   errorDeferred.resolve();
 };
 socket.onclose = (e) => {
+  console.log(e);
   assertEquals(e.reason, "No response from ping frame.");
   closeDeferred.resolve();
 };
@@ -22,4 +24,6 @@ await respondWith(response);
 
 await errorDeferred.promise;
 await closeDeferred.promise;
-listener.close();
+
+// TODO(mmastrac): this doesn't exit on its own. Why?
+Deno.exit(123);