Make sure += follows accessing, rather than setting

Author: A5rocks

src/trio/_tests/test_timeouts.py

@@ -249,6 +249,8 @@ def test_invalid_access_unentered() -> None:
     match_str = "^Unentered relative cancel scope does not have an absolute deadline"
     with pytest.raises(RuntimeError, match=match_str):
         assert cs.deadline
+    with pytest.raises(RuntimeError, match=match_str):
+        cs.deadline += 5
 
     # switch from relative to absolute
     cs.deadline = 7
@@ -260,6 +262,8 @@ def test_invalid_access_unentered() -> None:
     )
     with pytest.raises(RuntimeError, match=match_str):
         assert cs.relative_deadline
+    with pytest.raises(RuntimeError, match=match_str):
+        cs.relative_deadline += 5
 
     cs.relative_deadline = 9
     assert cs.relative_deadline == 9