selftests/cpu-hotplug: Skip test when there is only one online cpu

For only one online cpu case, 'make run_tests' try to offline the cpu0 that will
always fail since the host can't offline this unique online cpu.

this patch will skip the test to avoid this failure.

Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>