tests: posix: pthread: test for pthread descriptor leaks

Add a simple test to ensure that we can create and join a
single thread `CONFIG_MAX_PTHREAD_COUNT` * 2 times. If
there are leaks, then `pthread_create()` should
eventually return `EAGAIN`. If there are no leaks, then
the test should pass.

Fixes #47609

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