feature: allow testing cp38-macosx_arm64 wheel

Per discussion in https://github.com/pypa/cibuildwheel/pull/1169, the default installer used for cp38 is an Intel installer.
It makes sense to skip testing arm64 wheels in this case.

However, if the user choose to manually install the universal2 CPython version, then, tests shall be run on arm64.

This allows users that either target 11.0+ on intel, universal2 or only build for arm64 to get the arm64 wheel tested on AppleSilicon.

test/conftest.py

@@ -8,6 +8,12 @@ def pytest_addoption(parser) -> None:
         "--run-emulation", action="store_true", default=False, help="run emulation tests"
     )
     parser.addoption("--run-podman", action="store_true", default=False, help="run podman tests")
+    parser.addoption(
+        "--run-cp38-universal2",
+        action="store_true",
+        default=False,
+        help="macOS cp38 uses the universal2 installer",
+    )
 
 
 @pytest.fixture(