Setup tests

- 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.

Justfile

@@ -7,3 +7,12 @@ godot := `echo "${GODOT:-godot}"`
 
 build:
     cd addons/godot_xterm/native && scons
+
+install:
+    {{godot}} --headless -s plug.gd install
+
+test:
+    {{godot}} --headless -s addons/gut/gut_cmdln.gd -gdir=res://test -gexit
+
+uninstall:
+    {{godot}} --headless -s plug.gd uninstall