selftests: user: return Kselftest Skip code for skipped tests

When user 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. Add an explicit check
for module presence and return skip code if module isn't present.

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>