selftests: pstore: return Kselftest Skip code for skipped tests

When pstore_post_reboot test gets 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.

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