fix(test): check warning type in tests

The message returned by `pytest.warns` can now be `str`, so make sure
that it is in fact `Warning`. This fixes a mypy error.

tests/challenge/test_config.py

@@ -114,6 +114,7 @@ def test_warn_multiline_flag(configloader, test_datadir) -> None:
         cfg, errors = configloader.check_config(test_datadir / "challenge.yml")
     assert errors is None
     assert len(record) == 1
+    assert isinstance(record[0].message, Warning)
     assert (
         str(record[0].message.args[0])
         == "Flag contains multiple lines; is this intended?"

tests/project/test_assets.py

@@ -306,6 +306,7 @@ def test_extra_file(self, datadir: Path, am_fn: assets.AssetManager) -> None:
         with pytest.warns(RuntimeWarning) as record:
             ctx.sync(check=True)
         assert len(record) == 1
+        assert isinstance(record[0].message, Warning)
         assert "Unexpected item found in cache: " in str(record[0].message.args[0])
         assert not file1.exists()
 
@@ -317,6 +318,7 @@ def test_extra_dir(self, datadir: Path, am_fn: assets.AssetManager) -> None:
         with pytest.warns(RuntimeWarning) as record:
             ctx.sync(check=True)
         assert len(record) == 1
+        assert isinstance(record[0].message, Warning)
         assert "Unexpected item found in cache: " in str(record[0].message.args[0])
         assert not dir1.exists()