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
```

.pre-commit-config.yaml

@@ -40,3 +40,10 @@ repos:
         language: system
         entry: bazel run //swift/codegen
         pass_filenames: false
+
+      - id: swift-codegen-unit-tests
+        name: Run Swift code generation unit tests
+        files: ^swift/codegen
+        language: system
+        entry: bazel test //swift/codegen:tests
+        pass_filenames: false