tests: posix: common: split syslog tests into standalone test #80986
Description
Is your enhancement proposal related to a problem? Please describe.
Make the syslog-related tests in tests/posix/common into a standalone test.
Describe the solution you'd like
The obvious way to group syslog-related testsuites is by XSI_SYSTEM_LOGGING Option Group.
Describe alternatives you've considered
The way the testsuite has been designed historically.
Additional context
See #79454 for more info