rcutorture: Print grace-period performance statistics

Sometime problems can manifest themselves as unusually slow grace periods.
This commit therefore prints the number of rcutorture updates during the
test and the number per second.  These statistics are harvested from the
config.out and qemu-cmd files, and are silently omitted if these files
are not available, as would be the case if there was a build failure or
a boot-time hang.

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