selftests: timers: Fix run_destructive_tests target to handle skipped tests

When a test exits with skip exit code of 4, "make run_destructive_tests"
halts testing. Fix run_destructive_tests target to handle error exit codes.

Cc: stable <stable@vger.kernel.org> [4.13+]
Reported-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>