Library SCons boilerplate to build projects.

Works by executing project `SConstruct`s file in a cloned env (a bit
like Godot does for modules) so you don't have to worry about platform
and toolchain setup.

Convert the project test file to work as submodule, add it to CI

Run with:

```
scons build_projects=test,/path/to/other/project
```

.github/workflows/ci.yml

@@ -152,6 +152,10 @@ jobs:
       #    cd test
       #    scons target=release use_mingw=yes -j $env:NUMBER_OF_PROCESSORS
 
+      - name: Build test
+        run: |
+          scons platform=${{ matrix.platform }} target=release ${{ matrix.flags }} -j2 build_projects=test
+
       - name: Upload artifact
         uses: actions/upload-artifact@v2
         with: