tools/rcutorture: Fix BUG parsing of console.log

For the rcutorture test summary log file console.log of virtual machines is
parsed. When a console.log contains "DEBUG", BUG counter is incremented
because regular expression does not handle to ignore DEBUG.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Reviewed-by: Benedikt Spranger <b.spranger@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>