Keep platform-default tests independent of a running host installation.
These tests simulate macOS, but discovery otherwise finds the real Linux process.

--- a/tests/test_platform_paths.py
+++ b/tests/test_platform_paths.py
@@ -27,6 +27,11 @@
 
 
 class ResolvePathEnvOverrideTest(unittest.TestCase):
+    def setUp(self):
+        discovery = patch.object(platform_utils, "discover_scripting_lib", return_value=None)
+        discovery.start()
+        self.addCleanup(discovery.stop)
+
     def test_defaults_used_when_env_unset(self):
         with patch.object(platform_utils, "get_platform", return_value="darwin"), \
                 patch.dict(os.environ, _env_without_overrides(), clear=True):