testing: write a test to validate that custom attributes can flow through the display configuration

[mattkae]

Validate that custom attributes can be provided in the display configuration. This
was first encountered in #172. To
test:

1. Start a Mir compositor with the --display-config static=$FILENAME flag
2. In the configuration, have a custom attribute, e.g. snap-name in the following example:

   # ...
    HDMI-A-4:
      state: enabled
      mode: [email protected]
      position: [2560, 0]
      orientation: normal
      scale: 1
      group: 0
      snap-name: mir-kiosk-neverputt

3. Check the logs of your compositor. On ubuntu-frame you can do:

   snap logs ubuntu-frame -n 50

4. Validate that you see your custom attribute when the display is logged