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

.github/workflows/test.yml

@@ -83,7 +83,7 @@ jobs:
           CIBW_ENABLE=all
         else
           # get the default CIBW_ENABLE value from the test module
-          CIBW_ENABLE=$(uv run --no-sync python -c 'import sys, test.conftest as c; sys.stdout.write(c.DEFAULT_CIBW_ENABLE)')
+          CIBW_ENABLE=$(uv run --no-sync python -c 'import sys, test.utils as c; sys.stdout.write(c.DEFAULT_CIBW_ENABLE)')
 
           # if this is a PR, check for labels
           if [[ -n "$GITHUB_PR_LABEL_CI_PYPY" ]]; then