selftests: kselftest framework: change skip exit code to 0

When a test is skipped, instead of using a special exit code of 4, treat
it as pass condition and use exit code of 0. It makes sense to treat skip
as pass since the test couldn't be run as opposed to a failed test.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>