Install dejavu fonts for Cockpit tests

The font Firefox uses when we don't ensure dejavu is installed
seems to bounce around a bit, so let's ensure the dejavu fonts
are there before we start Firefox. Also update a needle for
this.

needles/cockpit/logs_entry-20171010.json

@@ -0,0 +1,15 @@
+{
+  "area": [
+    {
+      "type": "match",
+      "width": 43,
+      "height": 13,
+      "ypos": 216,
+      "xpos": 794
+    }
+  ],
+  "tags": [
+    "cockpit_logs_entry"
+  ],
+  "properties": []
+}

tests/server_cockpit_default.pm

@@ -15,6 +15,8 @@ sub run {
     assert_script_run 'dnf -y groupinstall "base-x"', 300;
     # FIXME: this should probably be in base-x...X seems to fail without
     assert_script_run 'dnf -y install libglvnd-egl', 120;
+    # try to avoid random weird font selection happening
+    assert_script_run 'dnf -y install dejavu-sans-fonts dejavu-sans-mono-fonts dejavu-serif-fonts', 120;
     assert_script_run 'dnf -y install firefox', 120;
     start_cockpit(0);
     # quit firefox (return to console)