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/test_schema.py

@@ -0,0 +1,158 @@
+import io
+import pathlib
+import sys
+
+import mock
+import pytest
+
+import swift.codegen.lib.schema as schema
+from swift.codegen.test.utils import *
+
+root_name = schema.root_class_name
+
+@pytest.fixture
+def load(tmp_path):
+    file = tmp_path / "schema.yml"
+    def ret(yml):
+        write(file, yml)
+        return schema.load(file)
+
+    return ret
+
+def test_empty_schema(load):
+    ret = load("{}")
+    assert ret.classes == [schema.Class(root_name)]
+    assert ret.includes == set()
+
+
+def test_one_empty_class(load):
+    ret = load("""
+MyClass: {}
+""")
+    assert ret.classes == [
+        schema.Class(root_name, derived={'MyClass'}),
+        schema.Class('MyClass', bases={root_name}),
+    ]
+
+
+def test_two_empty_classes(load):
+    ret = load("""
+MyClass1: {}
+MyClass2: {}
+""")
+    assert ret.classes == [
+        schema.Class(root_name, derived={'MyClass1', 'MyClass2'}),
+        schema.Class('MyClass1', bases={root_name}),
+        schema.Class('MyClass2', bases={root_name}),
+    ]
+
+
+def test_two_empty_chained_classes(load):
+    ret = load("""
+MyClass1: {}
+MyClass2:
+    _extends: MyClass1
+""")
+    assert ret.classes == [
+        schema.Class(root_name, derived={'MyClass1'}),
+        schema.Class('MyClass1', bases={root_name}, derived={'MyClass2'}),
+        schema.Class('MyClass2', bases={'MyClass1'}),
+    ]
+
+
+def test_empty_classes_diamond(load):
+    ret = load("""
+A: {}
+B: {}
+C:
+    _extends: 
+        - A
+        - B
+""")
+    assert ret.classes == [
+        schema.Class(root_name, derived={'A', 'B'}),
+        schema.Class('A', bases={root_name}, derived={'C'}),
+        schema.Class('B', bases={root_name}, derived={'C'}),
+        schema.Class('C', bases={'A', 'B'}),
+    ]
+
+
+def test_dir(load):
+    ret = load("""
+A:
+    _dir: other/dir
+""")
+    assert ret.classes == [
+        schema.Class(root_name, derived={'A'}),
+        schema.Class('A', bases={root_name}, dir=pathlib.Path("other/dir")),
+    ]
+
+
+def test_directory_filter(load):
+    ret = load("""
+_directories:
+    first/dir: '[xy]'
+    second/dir: foo$
+    third/dir: bar$
+Afoo: {}
+Bbar: {}
+Abar: {}
+Bfoo: {}
+Ax: {}
+Ay: {}
+A: {}
+""")
+    assert ret.classes == [
+        schema.Class(root_name, derived={'Afoo', 'Bbar', 'Abar', 'Bfoo', 'Ax', 'Ay', 'A'}),
+        schema.Class('Afoo', bases={root_name}, dir=pathlib.Path("second/dir")),
+        schema.Class('Bbar', bases={root_name}, dir=pathlib.Path("third/dir")),
+        schema.Class('Abar', bases={root_name}, dir=pathlib.Path("third/dir")),
+        schema.Class('Bfoo', bases={root_name}, dir=pathlib.Path("second/dir")),
+        schema.Class('Ax', bases={root_name}, dir=pathlib.Path("first/dir")),
+        schema.Class('Ay', bases={root_name}, dir=pathlib.Path("first/dir")),
+        schema.Class('A', bases={root_name}, dir=pathlib.Path()),
+    ]
+
+
+def test_directory_filter_override(load):
+    ret = load("""
+_directories:
+    one/dir: ^A$
+A:
+    _dir: other/dir
+""")
+    assert ret.classes == [
+        schema.Class(root_name, derived={'A'}),
+        schema.Class('A', bases={root_name}, dir=pathlib.Path("other/dir")),
+    ]
+
+
+def test_lowercase_rejected(load):
+    with pytest.raises(AssertionError):
+        load("aLowercase: {}")
+
+
+def test_digit_rejected(load):
+    with pytest.raises(AssertionError):
+        load("1digit: {}")
+
+
+def test_properties(load):
+    ret = load("""
+A:
+     one: string
+     two: int?
+     three: bool*
+""")
+    assert ret.classes == [
+        schema.Class(root_name, derived={'A'}),
+        schema.Class('A', bases={root_name}, properties=[
+            schema.SingleProperty('one', 'string'),
+            schema.OptionalProperty('two', 'int'),
+            schema.RepeatedProperty('three', 'bool'),
+        ]),
+    ]
+
+
+if __name__ == '__main__':
+    sys.exit(pytest.main())