tests: lib: c_lib: separate libc tests into smaller suites

C library testing is mainly there to support what is
necessary to support Zephyr. We do test a variety of libcs
currently, which is where YAML comes in handy.

However, the main libc testsuite can be overkill for testing
some things, and might not be suitable for testing optional
features.

Create a 'common' subdirectory for common libc tests.

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