selftests: ktap_helpers: Add helper to pass/fail test based on exit code

Similar to the C counterpart, add a helper function that runs a command
and passes or fails the test based on the result.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>