selftests/memory-hotplug: Add checking after online or offline

Add checking for online_memory_expect_success()/
offline_memory_expect_success()/offline_memory_expect_fail(), or
the test would exit 0 although the functions return 1.

Signed-off-by: Zhao Gongyi <zhaogongyi@huawei.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>