rcutorture: Disable RCU priority boost testing on non-TREE03 scenarios

The TREE03 scenario has been configured to permit testing of RCU
priority boosting, including setting RCU kthead priorities to 2.
But now that RCU priority boosting is default for preemptible kernels,
it is now automatically tested for TREE01, TREE02, TREE08, and TREE09,
none of which have RCU kthread priorities set up for such testing.
One approach would be to set up these scenarios' kthread priorities,
but this would fail to test common-case kernels having default priorities.

This commit therefore disables the testing of RCU priority boosting
except on scenario TREE03, which is set up properly for this purpose,
though rather unrealistically.

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