Rename main_util_fixtures.py to conftest.py to allow fixture auto discovery

Also fix new PEP-008 issues.

unit_test/main_tests/main_platform_test.py

@@ -0,0 +1,59 @@
+import sys
+
+import pytest
+
+from cibuildwheel.__main__ import main
+
+from conftest import MOCK_PROJECT_DIR  # noqa: I100
+
+
+def test_unknown_platform_non_ci(monkeypatch, capsys):
+    monkeypatch.delenv('CI', raising=False)
+    monkeypatch.delenv('BITRISE_BUILD_NUMBER', raising=False)
+    monkeypatch.delenv('AZURE_HTTP_USER_AGENT', raising=False)
+
+    with pytest.raises(SystemExit) as exit:
+        main()
+        assert exit.value.code == 2
+    _, err = capsys.readouterr()
+
+    assert 'cibuildwheel: Unable to detect platform.' in err
+    assert 'cibuildwheel should run on your CI server' in err
+
+
+def test_unknown_platform_on_ci(monkeypatch, capsys):
+    monkeypatch.setenv('CI', 'true')
+    monkeypatch.setattr(sys, 'platform', 'nonexistent')
+
+    with pytest.raises(SystemExit) as exit:
+        main()
+        assert exit.value.code == 2
+    _, err = capsys.readouterr()
+
+    assert 'cibuildwheel: Unable to detect platform from "sys.platform"' in err
+
+
+def test_unknown_platform(monkeypatch, capsys):
+    monkeypatch.setenv('CIBW_PLATFORM', 'nonexistent')
+
+    with pytest.raises(SystemExit) as exit:
+        main()
+    _, err = capsys.readouterr()
+
+    assert exit.value.code == 2
+    assert 'cibuildwheel: Unsupported platform: nonexistent' in err
+
+
+def test_platform_argument(platform, intercepted_build_args, monkeypatch):
+    monkeypatch.setenv('CIBW_PLATFORM', 'nonexistent')
+    monkeypatch.setattr(sys, 'argv', sys.argv + ['--platform', platform])
+
+    main()
+
+    assert intercepted_build_args.kwargs['project_dir'] == MOCK_PROJECT_DIR
+
+
+def test_platform_environment(platform, intercepted_build_args, monkeypatch):
+    main()
+
+    assert intercepted_build_args.kwargs['project_dir'] == MOCK_PROJECT_DIR