tests: posix: common: mqueue: remove overspecified pthread_attr_t

Much of tests/posix/common still overspecifies pthread_attr_t
options.

Default thread attributes are perfectly fine for the vast
majority of spawned threads in this testsuite, so simply use
a NULL pthread_attr_t* argument.

This fixes piles of bugs because we have not properly used
pthread_attr_destroy() appropriately.

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