Add a `debug_mode`-state reversion test

tests/test_root_runtime.py

@@ -83,3 +83,26 @@ def test_implicit_root_via_first_nursery(
             assert tractor.current_actor().aid.name == 'root'
 
     trio.run(main)
+
+
+def test_runtime_vars_unset(
+    reg_addr: tuple,
+    debug_mode: bool
+):
+    '''
+    Ensure any `._state._runtime_vars` are restored to default values
+    after the root actor-runtime exits!
+
+    '''
+    assert not tractor._state._runtime_vars['_debug_mode']
+    async def main():
+        assert not tractor._state._runtime_vars['_debug_mode']
+        async with tractor.open_nursery(
+            debug_mode=True,
+        ):
+            assert tractor._state._runtime_vars['_debug_mode']
+
+        # after runtime closure, should be reverted!
+        assert not tractor._state._runtime_vars['_debug_mode']
+
+    trio.run(main)