Factor DM login process out of _graphical_wait_login

There are several other tests doing the same thing (but not as
safely, in some cases). To improve reliability and reduce
duplication, let's factor this out into utils.pm and reuse it
where appropriate.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

needles/gnome/apps/clocks/clocks_alarm_hour-active-20240926.json

@@ -1,15 +0,0 @@
-{
-  "area": [
-    {
-      "height": 36,
-      "type": "match",
-      "ypos": 217,
-      "xpos": 409,
-      "width": 47
-    }
-  ],
-  "properties": [],
-  "tags": [
-    "clocks_alarm_hour"
-  ]
-}

needles/kde/desktop_expand_systray-darkgrey-20240926.json

@@ -1,16 +0,0 @@
-{
-  "area": [
-    {
-      "width": 17,
-      "xpos": 880,
-      "ypos": 729,
-      "type": "match",
-      "height": 18
-    }
-  ],
-  "properties": [],
-  "tags": [
-    "DESKTOP-kde",
-    "desktop_expand_systray"
-  ]
-}