lib/test_cpumask: drop cpu_possible_mask full test

When the number of CPUs that can possibly be brought online is known at
boot time, e.g. when HOTPLUG is disabled, nr_cpu_ids may be smaller than
NR_CPUS. In that case, cpu_possible_mask would not be completely filled,
and cpumask_full(cpu_possible_mask) can return false for valid system
configurations.

Without this test, cpu_possible_mask contents are still constrained by
a check on cpumask_weight(), as well as tests in test_cpumask_first(),
test_cpumask_last(), test_cpumask_next(), and test_cpumask_iterators().

Fixes: c41e8866

 ("lib/test: introduce cpumask KUnit test suite")
Link: https://lore.kernel.org/lkml/346cb279-8e75-24b0-7d12-9803f2b41c73@riseup.net/
Reported-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Sander Vanheule <sander@svanheule.net>
Tested-by: Maíra Canal <mairacanal@riseup.net>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>