Prevent useless logging in the "tcp" system test

Prevent useless logging in the "tcp" system test

The regression test for CVE-2020-8620 causes a lot of useless messages
to be logged.  However, globally decreasing the log level for the
affected named instance would be a step too far as debugging information
may be useful for troubleshooting other checks in the "tcp" system test.
Starting a separate named instance for a single check should be avoided
when possible and thus is also not a good solution.  As a compromise,
run "rndc trace 1" for the affected named instance before starting the
regression test for CVE-2020-8620.