kunit, slub: add test_kfree_rcu() and test_leak_destroy()

Add a test that will create cache, allocate one object, kfree_rcu() it
and attempt to destroy it. As long as the usage of kvfree_rcu_barrier()
in kmem_cache_destroy() works correctly, there should be no warnings in
dmesg and the test should pass.

Additionally add a test_leak_destroy() test that leaks an object on
purpose and verifies that kmem_cache_destroy() catches it.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>