selftests: memory-hotplug: silence test command echo

Silence the following command being printed while running test.

./mem-on-off-test.sh -r 2 && echo "selftests: memory-hotplug [PASS]" ||
echo "selftests: memory-hotplug [FAIL]"

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>