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 02d8f075)
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>

tools/testing/selftests/memory-hotplug/mem-on-off-test.sh

@@ -207,6 +207,7 @@ prerequisite_extra()
 	fi
 }
 
+echo -e "\t Test with memory notifier error injection"
 prerequisite_extra
 
 #