rcutorture: Test partial nohz_full= configuration

The current set of tests covers only cases where either all possible CPUs
are nohz_full= CPUs or none of them are.  Because there have been some
recent bug escapes in cases where only some of the CPUs are nohz_full=
CPUs, this commit add a configuration where only half of the CPUs are
nohz_full= CPUs.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>