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_container_engine.py

@@ -6,7 +6,7 @@ basic_project = test_projects.new_c_project()
 
 
 def test_podman(tmp_path, capfd, request):
-    if utils.platform != "linux":
+    if utils.get_platform() != "linux":
         pytest.skip("the test is only relevant to the linux build")
 
     if not request.config.getoption("--run-podman"):
@@ -36,7 +36,7 @@ def test_podman(tmp_path, capfd, request):
 
 
 def test_create_args(tmp_path, capfd):
-    if utils.platform != "linux":
+    if utils.get_platform() != "linux":
         pytest.skip("the test is only relevant to the linux build")
 
     project_dir = tmp_path / "project"