tests: posix: stress test for pthread_create and pthread_join

Recently, a race condition was discovered in `pthread_create()`
and `pthread_join()` that would trigger an assertion in
`kernel/sched.c` of the form below.
```
aborted _current back from dead
```

This was mainly observed in Qemu. Unfortunately, Qemu SMP
platforms exhibit a real and measurable "scheduler noise"
which makes testing rather difficult.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>