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

tools/testing/selftests/cpu-hotplug/cpu-on-off-test.sh

@@ -28,6 +28,12 @@ prerequisite()
 	echo "CPU online/offline summary:"
 	online_cpus=`cat $SYSFS/devices/system/cpu/online`
 	online_max=${online_cpus##*-}
+
+	if [[ "$online_cpus" = "$online_max" ]]; then
+		echo "$msg: since there is only one cpu: $online_cpus"
+		exit 0
+	fi
+
 	echo -e "\t Cpus in online state: $online_cpus"
 
 	offline_cpus=`cat $SYSFS/devices/system/cpu/offline`