tests: posix: common: split c_lib_ext tests into standalone test #81488
Description
Is your enhancement proposal related to a problem? Please describe.
Make the c_lib_ext-related tests in tests/posix/common into a standalone test.
Describe the solution you'd like
The obvious way to group c_lib_ext-related testsuites is by POSIX_C_LIB_EXT Option Group.
Describe alternatives you've considered
The way the testsuite has been designed historically.
Additional context
See #79454 for more info