torture: Exclude "NOHZ tick-stop error" from fatal errors

The "NOHZ tick-stop error: Non-RCU local softirq work is pending"
warning happens frequently and appears to be irrelevant to the various
torture tests.  This commit therefore filters it out.

If there proves to be a need to pay attention to it a later commit will
add an "advice" category to allow the user to immediately see that
although something happened, it was not an indictment of the system
being tortured.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>