sim; testsuite: allow tests to set no output

sim; testsuite: allow tests to set no output

If a test doesn't write anything at all to stdout, the current test
framework can't support that.  Even if you put a blank output line:
	# output:
the setup happily clobbers that with a default pass/fail string.

Tweak the parsing logic so we only set the output to pass/fail when
the test has no output marker.