selftests/harness: Actually report SKIP for signal tests

Tests that were expecting a signal were not correctly checking for a
SKIP condition. Move the check before the signal checking when
processing test result.

Cc: Shuah Khan <shuah@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Will Drewry <wad@chromium.org>
Cc: linux-kselftest@vger.kernel.org
Fixes: 9847d24a ("selftests/harness: Refactor XFAIL into SKIP")
Signed-off-by: Kees Cook <keescook@chromium.org>