include some grey in the reboot button match

Summary:
There's a weird problem that we hit occasionally only on the
Server image, see e.g.:
https://openqa.fedoraproject.org/tests/14300
I *think* what's happening (thanks hergertme from #gtk+ IRC for
help with this) is that when the 'install done!' text and the
reboot button appears, there's a short transition for the button
so it doesn't look *exactly* like the screenshot right away. If
openQA's needle match loop happens to kick in and check the
match right after the text and button appear, while the
transition is happening, it decides that some random area of the
gradient down the left hand side of the screen matches *better*
than the button, so it 'clicks' that, and we don't get a reboot.
If the openQA needle match loop happens to kick in a millisecond
or whatever later, after the button is done transitioning, the
button is now a 100% match and the bug doesn't happen. This only
happens on Server because the left-hand side gradient differs in
each flavor, and only the Server one just happens to have a spot
which is nearly identical to the needle area.

So to avoid that, let's include a bit of the grey space next to
the button. This is a bit more fragile against GTK+ changes to
button design or padding or whatever, but it will at least avoid
the match issue.

The other option would be to try and hit the button with tab and
enter or something, I guess.

Test Plan:
bit hard since this is an intermittent bug, I guess
run install_default on a Server image a bunch of times and see if
it always works.

Reviewers: jskladan, garretraziel

Reviewed By: garretraziel

Subscribers: tflink

Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D824

needles/anaconda_install_done.json

@@ -1,24 +1,24 @@
 {
-  "area": [
-    {
-      "xpos": 695,
-      "ypos": 632,
-      "height": 20,
-      "width": 325,
-      "type": "match"
-    },
-    {
-      "ypos": 678,
-      "height": 28,
-      "xpos": 1000,
-      "type": "match",
-      "width": 17
-    }
-  ],
-  "tags": [
-    "anaconda_install_done",
-    "ENV-DISTRI-fedora",
-    "LANGUAGE-english"
-  ],
-  "properties": []
+    "area": [
+        {
+            "height": 20,
+            "type": "match",
+            "width": 325,
+            "xpos": 695,
+            "ypos": 632
+        },
+        {
+            "height": 28,
+            "type": "match",
+            "width": 23,
+            "xpos": 1000,
+            "ypos": 678
+        }
+    ],
+    "properties": [],
+    "tags": [
+        "anaconda_install_done",
+        "ENV-DISTRI-fedora",
+        "LANGUAGE-english"
+    ]
 }