selftests: kmod: return Kselftest Skip code for skipped tests

When kmod test is skipped because of unmet dependencies and/or unsupported
configuration, it returns 0 which is treated as a pass by the Kselftest
framework. This leads to false positive result even when the test could
not be run. It returns fail in some cases when test is skipped. Either way,
it is incorrect and incosnistent reporting.

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

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>
Reviewed-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>