ztest_resume should be ztest_resume_thread in pthread_create()

ztest_resume should be ztest_resume_thread in pthread_create()

Fix an accidental bug introducted by the pthreads changes.  When creating
a resume thread the handler function was accidentally changed from
ztest_resume to ztest_resume_thread.  There's a decent chance this
may explain some/all of the crashes that have be observed running
ztest.  This issue was exposed and easily fixed once the -Wno-unused
check was removed from the build system.  Happily at least one real
bug fix resulted from that cleanup.