test-standalone: Use a realistic signal handler

If we use None as a signal handler, the test will log unhandled exception
tracebacks every time a match rule matches a signal, because None isn't
callable.

Fixes: d159c611 "test: Add basic coverage for arg0namespace"
Fixes: cd745679 "test: Add basic coverage for argNpatch matches"
Signed-off-by: Simon McVittie <smcv@collabora.com>