tests: posix: common: add a small delay in pthread_descriptor_leak

The `aborted _current back from dead` error may appear in this
particular test under Qemu (and in particular Qemu SMP) systems.

A small delay between `pthread_create()` and `pthread_join()`
is sufficient to mitigate the issue.

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