Add timeout guard around caller side context open

tests/test_context_stream_semantics.py

@@ -265,6 +265,7 @@ async def test_callee_closes_ctx_after_stream_open():
             enable_modules=[__name__],
         )
 
+        with trio.fail_after(2):
             async with portal.open_context(
                 close_ctx_immediately,
 
@@ -297,6 +298,7 @@ async def test_callee_closes_ctx_after_stream_open():
                 # of a stream to the context (at least until a time of
                 # if/when we decide that's a good idea?)
                 try:
+                    with trio.fail_after(0.5):
                         async with ctx.open_stream() as stream:
                             pass
                 except trio.ClosedResourceError: