testsuite: fix exiting with success on no output activity

testsuite: fix exiting with success on no output activity

If we were expecting output on stdout or stderr but the test didn't
produce any, we were incorrectly assuming the test was successful.
Now test on exit if there was activity on the monitored fd. If there
was, check also if the file size to check for output is > 0 for the
cases in which we want to assert there was no activity on certain fd.