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
```

swift/codegen/test/utils.py

@@ -0,0 +1,50 @@
+import pathlib
+from unittest import mock
+
+import pytest
+
+from swift.codegen.lib import render, schema
+
+schema_dir = pathlib.Path("a", "dir")
+schema_file = schema_dir / "schema.yml"
+
+
+def write(out, contents=""):
+    out.parent.mkdir(parents=True, exist_ok=True)
+    with open(out, "w") as out:
+        out.write(contents)
+
+
+@pytest.fixture
+def renderer():
+    return mock.Mock(spec=render.Renderer())
+
+
+@pytest.fixture
+def opts():
+    return mock.MagicMock()
+
+
+@pytest.fixture(autouse=True)
+def override_paths(tmp_path):
+    with mock.patch("swift.codegen.lib.paths.swift_dir", tmp_path):
+        yield
+
+
+@pytest.fixture
+def input(opts, tmp_path):
+    opts.schema = tmp_path / schema_file
+    with mock.patch("swift.codegen.lib.schema.load") as load_mock:
+        load_mock.return_value = schema.Schema([])
+        yield load_mock.return_value
+    assert load_mock.mock_calls == [
+        mock.call(opts.schema)
+    ], load_mock.mock_calls
+
+
+def run_generation(generate, opts, renderer):
+    output = {}
+
+    renderer.render.side_effect = lambda data, out: output.__setitem__(out, data)
+    generate(opts, renderer)
+    return output