Improve integration test options (#2385)

* Improve integration test options

- Keep the platform and enable options in os.environ, don't duplicate state in utils
- Provide more user-friendly options for setting enable and platform in integration tests

* Ensure that CIBW_ENABLE is set in the test process

* Use get_enable_groups() where necessary

test/test_windows.py

@@ -39,7 +39,7 @@ def skip_if_no_msvc(arm64: bool = False) -> None:
 
 @pytest.mark.parametrize("use_pyproject_toml", [True, False])
 def test_wheel_tag_is_correct_when_using_windows_cross_compile(tmp_path, use_pyproject_toml):
-    if utils.platform != "windows":
+    if utils.get_platform() != "windows":
         pytest.skip("This test is only relevant to Windows")
 
     skip_if_no_msvc(arm64=True)