Rename 'nursery' fixture to 'test_nursery'

Every nursery is called 'nursery'. This is less generic, and
emphasizes that this nursery is special, because its lifetime is
linked to the test.

Kept 'nursery' around for now as a deprecated alias.

pytest_trio/_tests/test_nursery_fixture.py

@@ -9,8 +9,8 @@ async def handle_client(stream):
 
 
 @pytest.fixture
-async def server(nursery):
-    listeners = await nursery.start(trio.serve_tcp, handle_client, 0)
+async def server(test_nursery):
+    listeners = await test_nursery.start(trio.serve_tcp, handle_client, 0)
     return listeners[0]
 
 

pytest_trio/plugin.py

@@ -272,5 +272,16 @@ def autojump_clock():
 
 
 @trio_fixture
-def nursery(request):
+def test_nursery(request):
     return request.node._trio_nursery
+
+
+@trio_fixture
+def nursery(test_nursery):
+    import warnings
+    warnings.warn(
+        FutureWarning(
+            "The 'nursery' fixture has been renamed to 'test_nursery'"
+        )
+    )
+    return test_nursery