It's very flaky for some unknown reason. Something we have
an extra EXITED event. I suspect some other test is causing this.

We saw the following failure:
```
TestThreadInstrumentation#test_thread_instrumentation [/tmp/ruby/v3/src/trunk-random3/test/-ext-/thread/test_instrumentation_api.rb:25]:
Expected 0..3 to include 4.
```

Which shouldn't happen unless somehow there was a leaked thread.

* Extracted some assertions.
* Assert counter values should be positive.

Recently `TestThreadInstrumentation#test_join_counters` often fails as

```
<[1, 1, 1]> expected but was
<[2, 2, 2]>.
```

Probably it seems that the thread is suspended more than once.
There may be no guarantee that the subject thread never be suspended
more than once.

[Bug #18900]

Thread#join and a few other codepaths are using native sleep as
a way to suspend the current thread. So we should call the relevant
hook when this happen, otherwise some thread may transition
directly from `RESUMED` to `READY`.

I suspect that sometimes on CI the last thread is prempted before eaching the exit hook
causing the test to flake. I can't find a good way to force it to run.

It previously failed with:

```
    1) Failure:
  TestThreadInstrumentation#test_thread_instrumentation_fork_safe [/home/runner/work/ruby/ruby/src/test/-ext-/thread/test_instrumentation_api.rb:50]:
  <5> expected but was
  <4>.
```

Suggesting one `EXIT` event wasn't fired or processed.

Adding an assetion on `Thead#status` may help figure out what is wrong.

[Feature #18339]

After experimenting with the initial version of the API I figured there is a need
for an exit event to cleanup instrumentation data. e.g. if you record data in a
{thread_id -> data} table, you need to free associated data when a thread goes away.