torture: Use elapsed time to detect hangs

The kvm-test-1-run.sh currently counts "sleep 1" commands to detect
hangs.  This can fail spectacularly on busy systems, where "sleep 1"
might take far longer than one second to complete.  This commit
therefore changes hang detection to use elapsed time measurements.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>