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_before_test.py

@@ -43,7 +43,7 @@ def test(tmp_path, build_frontend_env):
         '''python -c "import pathlib, sys; pathlib.Path('{project}/pythonprefix_bt.txt').write_text(sys.prefix)"''',
     ]
 
-    if utils.platform == "pyodide":
+    if utils.get_platform() == "pyodide":
         before_test_steps.extend(
             ["pyodide build {project}/dependency", "pip install --find-links dist/ spam"]
         )