selftests: Change memory on-off-test.sh name to be unique

cpu and memory hotplug scripts use the same name. Change
memory on-off-test.sh to mem-on-off-test.sh.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>