Fix/improve test_terminal_reporter_writer_attr

It did not actually test that the attribute gets set.
This also checks the stacklevel etc.

testing/deprecated_test.py

@@ -1,8 +1,10 @@
+import copy
 import inspect
 
 import pytest
 from _pytest import deprecated
 from _pytest import nodes
+from _pytest.config import Config
 
 
 @pytest.mark.filterwarnings("default")
@@ -32,7 +34,7 @@ def test_pytest_collect_module_deprecated(attribute):
         getattr(pytest.collect, attribute)
 
 
-def test_terminal_reporter_writer_attr(pytestconfig):
+def test_terminal_reporter_writer_attr(pytestconfig: Config) -> None:
     """Check that TerminalReporter._tw is also available as 'writer' (#2984)
     This attribute has been deprecated in 5.4.
     """
@@ -43,15 +45,22 @@ def test_terminal_reporter_writer_attr(pytestconfig):
     except ImportError:
         pass
     terminal_reporter = pytestconfig.pluginmanager.get_plugin("terminalreporter")
-    expected_tw = terminal_reporter._tw
+    original_tw = terminal_reporter._tw
 
-    with pytest.warns(pytest.PytestDeprecationWarning):
-        assert terminal_reporter.writer is expected_tw
+    with pytest.warns(pytest.PytestDeprecationWarning) as cw:
+        assert terminal_reporter.writer is original_tw
+    assert len(cw) == 1
+    assert cw[0].filename == __file__
 
-    with pytest.warns(pytest.PytestDeprecationWarning):
-        terminal_reporter.writer = expected_tw
-
-    assert terminal_reporter._tw is expected_tw
+    new_tw = copy.copy(original_tw)
+    with pytest.warns(pytest.PytestDeprecationWarning) as cw:
+        terminal_reporter.writer = new_tw
+        try:
+            assert terminal_reporter._tw is new_tw
+        finally:
+            terminal_reporter.writer = original_tw
+    assert len(cw) == 2
+    assert cw[0].filename == cw[1].filename == __file__
 
 
 @pytest.mark.parametrize("plugin", sorted(deprecated.DEPRECATED_EXTERNAL_PLUGINS))