The default test grace period of two minutes is insufficient in some
cases and excessive in others.  This commit therefore increases the
default to three minutes, but also adds a --shutdown-grace parameter
to allow the default to be overridden.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>