torture: Suppress torture.sh "Zero time" messages for disabled tests

torture: Suppress torture.sh "Zero time" messages for disabled tests

The torture.sh script prints " --- Zero time for locktorture, disabling"
when the --duration parameter is too short to allow the test to run
even when locktorture has been disabled, for example, via --do-none.
The same is true for scftorture and rcutorture.

This commit therefore suppresses this message when the corresponding
test has been disabled.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org>