Fail the "tcp" system test when ans6 fails

Make the "tcp" system test fail if the Python tool used for establishing TCP connections (ans6) logs a result different than "OK" after processing a command sent to it (as that means the tool was unable to successfully perform the requested action), with the exception of cleanup errors at the end of the test which can be safely ignored. Note that the tool not returning any result at all in 10 seconds is still a fatal error in all cases.

(See !2549 (comment 89283) for context.)

Closes #1308 (closed)