rcutorture: Exempt TREE01 from forward-progress testing

Because TREE01 can end up running more vCPUs that physical CPUs,
hammering these shortchanged CPUs with tight loops containing call_rcu()
invocations seems a bit like overkill.  This commit therefore exempts
TREE01 from rcutorture's forward-progress testing.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>