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/test_macos_archs.py

@@ -167,3 +167,33 @@ def test_cp38_arm64_testing(tmp_path, capfd):
     expected_wheels = [w for w in ALL_MACOS_WHEELS if "cp38" in w]
 
     assert set(actual_wheels) == set(expected_wheels)
+
+
+def test_cp38_arm64_testing_universal2_installer(tmp_path, capfd, request):
+    if not request.config.getoption("--run-cp38-universal2"):
+        pytest.skip("needs --run-cp38-universal2 option to run")
+
+    project_dir = tmp_path / "project"
+    basic_project.generate(project_dir)
+
+    actual_wheels = utils.cibuildwheel_run(
+        project_dir,
+        add_env={
+            "CIBW_BUILD": "cp38-*",
+            "CIBW_TEST_COMMAND": '''python -c "import platform; print('running tests on ' + platform.machine())"''',
+            "CIBW_ARCHS": "x86_64,universal2,arm64",
+            "MACOSX_DEPLOYMENT_TARGET": "11.0",
+        },
+    )
+
+    captured = capfd.readouterr()
+
+    assert "running tests on x86_64" in captured.out
+    assert "running tests on arm64" in captured.out
+
+    warning_message = "While cibuildwheel can build CPython 3.8 universal2/arm64 wheels, we cannot test the arm64 part of them"
+    assert warning_message not in captured.err
+
+    expected_wheels = [w.replace("10_9", "11_0") for w in ALL_MACOS_WHEELS if "cp38" in w]
+
+    assert set(actual_wheels) == set(expected_wheels)