selftests/cpu-hotplug: Reserve one cpu online at least

Considering that we can not offline all cpus in any cases,
we need to reserve one cpu online when the test offline all
hotpluggable online cpus, otherwise the test will fail forever.

Fixes: d89dffa9 ("fault-injection: add selftests for cpu and memory hotplug")

Signed-off-by: Zhao Gongyi <zhaogongyi@huawei.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>