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_environment.py

@@ -64,7 +64,7 @@ def test_overridden_path(tmp_path, capfd):
     output_dir.mkdir()
 
     # mess up PATH, somehow
-    if utils.platform == "linux":
+    if utils.get_platform() == "linux":
         with pytest.raises(subprocess.CalledProcessError):
             utils.cibuildwheel_run(
                 project_dir,
@@ -128,7 +128,7 @@ def test_overridden_pip_constraint(tmp_path, build_frontend):
     )
     project.generate(project_dir)
 
-    if utils.platform == "linux":
+    if utils.get_platform() == "linux":
         # put the constraints file in the project directory, so it's available
         # in the docker container
         constraints_file = project_dir / "constraints.txt"