Swift: add unit tests to code generation

Tests can be run with
```
bazel test //swift/codegen:tests
```

Coverage can be checked installing `pytest-cov` and running
```
pytest --cov=swift/codegen swift/codegen/test
```

conftest.py

@@ -0,0 +1 @@
+# this empty file adds the repo root to PYTHON_PATH when running pytest