Don't attempt to test CPython 3.8 wheels on Apple Silicon. (#1171)

I've added a warning for users, so they know that it doesn't get tested.
See discussion in #1169 for the reason why.

Fixes #1168, fixes #1169

test/test_macos_archs.py

@@ -133,3 +133,36 @@ def test_universal2_testing(tmp_path, capfd, skip_arm64_test):
     expected_wheels = [w for w in ALL_MACOS_WHEELS if "cp39" in w and "universal2" in w]
 
     assert set(actual_wheels) == set(expected_wheels)
+
+
+def test_cp38_arm64_testing(tmp_path, capfd):
+    if utils.platform != "macos":
+        pytest.skip("this test is only relevant to macos")
+    if get_xcode_version() < (12, 2):
+        pytest.skip("this test only works with Xcode 12.2 or greater")
+    if platform.machine() != "arm64":
+        pytest.skip("this test only works on arm64")
+
+    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",
+        },
+    )
+
+    captured = capfd.readouterr()
+
+    assert "running tests on x86_64" in captured.out
+    assert "running tests on arm64" not 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 in captured.err
+
+    expected_wheels = [w for w in ALL_MACOS_WHEELS if "cp38" in w]
+
+    assert set(actual_wheels) == set(expected_wheels)