test: make full discovery optional-state independent

tests/test_wheel_runtime.py

@@ -40,8 +40,17 @@ class WheelRuntimeTests(unittest.TestCase):
                 "pip",
                 "install",
                 "--no-deps",
+                "--force-reinstall",
                 str(core_wheels[0]),
                 cwd=temporary_root,
+                # Full editable discovery exports every sibling src directory.
+                # Do not let pip mistake that importable Core metadata for an
+                # installation inside this fresh wheel-only environment.
+                env={
+                    key: value
+                    for key, value in os.environ.items()
+                    if key != "PYTHONPATH"
+                },
             )
 
             database_path = temporary_root / "wheel-runtime.db"