selftests: static_keys: return Kselftest Skip code for skipped tests

When static_keys test is skipped because of unmet dependencies and/or
unsupported configuration, it exits with error which is treated as a fail
by the Kselftest framework. This leads to false negative result even when
the test could not be run.

Change it to return kselftest skip code when a test gets skipped to clearly
report that the test could not be run.

Added an explicit searches for test_static_key_base and test_static_keys
modules and return skip code if they aren't found to differentiate between
the failure to load the module condition and module not found condition.

Kselftest framework SKIP code is 4 and the framework prints appropriate
messages to indicate that the test is skipped.

Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>