torture: Add a stop-run capability

When bisecting RCU issues, it is often the case that the first error in
an unsuccessful run will happen quickly, but that a successful run must
go on for some time in order to obtain a sufficiently low false-negative
error rate.  In many cases, a bisection requires multiple concurrent
runs, in which case the first failure in any run indicates failure,
pure and simple.  In such cases, it would speed things up greatly if
the first failure terminated all runs.

This commit therefore adds scripting that checks for a file named "STOP"
in the top-level results directory, terminating the run when it appears.
Note that in-progress builds will continue until completion, but future
builds and all runs will be cut short.

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