selftests: check hot-pluggagble memory for memory-hotplug test

Check for hot-pluggable memory availability in prerequisite() of the
memory-hotplug test.

Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

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

@@ -22,6 +22,11 @@ prerequisite()
 		echo $msg memory hotplug is not supported >&2
 		exit 0
 	fi
+
+	if ! grep -q 1 $SYSFS/devices/system/memory/memory*/removable; then
+		echo $msg no hot-pluggable memory >&2
+		exit 0
+	fi
 }
 
 #