selftests: add missing test name in memory-hotplug test

selftests: add missing test name in memory-hotplug test

BugLink: http://bugs.launchpad.net/bugs/1710868
There is no prompt for testing memory notifier error injection,
added with the same echo format of other tests above.

Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
(cherry picked from commit 02d8f075ac44f6f0dc46881965f815576921f2c0)
Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>