tests: eventfd: blocking read-write test for deadlock bug

Previously, the prototypical use case for `eventfd()` was not
possible in Zephyr due to a deadlock associated with the
`fdtable.c` `k_mutex`.

The prototypical use case for `eventfd()` is to make a
blocking call to `eventfd_read()` from one thread, and then
from another thread, call `eventfd_write()` to "wake up" the
reading thread.

This additional test case ensures that Zephyr supports the
prototypical `eventfd()` use case.

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