torture: Output per-failed-run summary lines from torture.sh

Currently, torture.sh lists the failed runs, but it is up to the user
to work out what failed.  This is especially annoying for KCSAN runs,
where RCU's tighter definitions result in failures being reported for
other parts of the kernel.  This commit therefore outputs "Summary:"
lines for each failed run, allowing the user to more quickly identify
which failed runs need focused attention.

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