- Install Gut using gd-plug.
- Add some basic tests.
- Update workflow to run tests.
- Add test and gd-plug install/uninstall recipes to Justfile.
- Re-enable debug builds as these are used by editor when testing.
Commit GUT directly to the git repo and remove gd-plug as it is no
longer required to install GUT.
Modify GUT to be used by command-line only.
For example:
```
cp test/.gutconfig.ci.json .gutconfig.json
godot --no-window -s addons/gut/gut_cmdln.gd
```
Keeps the archive small and licensing simpler.
Keep around the fancy fonts such as Noto Color Emoji and Unifont as they
are useful for testing.
With the xrdb import plugin making it easy to import themes, there is
little point in keeping other pre-defined themes around, so remove
these.
