tests: posix: common: separate xsi_threads_ext into standalone test

posix.common contains testsuites that can be separated into smaller
groups of tests. This change moves tests in the xsi_threads_ext
option group into a singular testsuite tests/posix/xsi_threads_ext
app directory.

Signed-off-by: Marvin Ouma <pancakesdeath@protonmail.com>