Add GUT (CLI only) and remove gd-plug

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
```

.github/workflows/main.yml

@@ -200,26 +200,6 @@ jobs:
           name: html5-gdnative-export-templates
           path: misc/export_templates/godot/bin/webassembly_gdnative_${{matrix.target}}.zip
 
-
-  install_plugins:
-    name: 'Install Plugins'
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v2
-      - name: Setup Godot
-        uses: lihop/setup-godot@v1.0.0
-      - name: Install plugins
-        run: godot --no-window -s plug.gd install
-      - name: Upload install plugins for use by other jobs
-        uses: actions/upload-artifact@v2
-        with:
-          name: plugins
-          retention-days: 1 # Minimum.
-          path: |
-            addons
-            !addons/godot_xterm
- 
-
   html5_export:
     name: 'HTML5 Export'
     needs: [ build_docker, build_native, export_template ]
@@ -240,8 +220,6 @@ jobs:
       with:
         name: html5-gdnative-export-templates
         path: misc/export_templates/godot/bin
-    - name: Install plugins
-      run: godot --no-window -s plug.gd install
     - name: Create export directory
       run: mkdir -p docs/demo
     - name: Export html5
@@ -257,7 +235,7 @@ jobs:
 
   test:
     name: 'Test'
-    needs: [ install_plugins, build_docker, build_native ]
+    needs: [ build_docker, build_native ]
     runs-on: ${{ matrix.os }}
     strategy:
       fail-fast: false
@@ -285,11 +263,6 @@ jobs:
         with:
           name: libgodot-xterm-release
           path: addons/godot_xterm/native/bin
-      - name: Install plugins
-        uses: actions/download-artifact@v2
-        with:
-          name: plugins
-          path: ./addons
       - name: Run tests
         if: ${{ matrix.godot_version != 'v3.2-stable' }}
         shell: bash