test_groupby: Clear recorded warnings for pytest-8 compatibility

Clear the warnings recorded during the `pytest.warns()` use
in `test_groupby_dims_property`, to fix test failures with pytest-8.0.0.
Prior to this version, `pytest.warns()` invocation used to capture all
warnings.  Now it only captures the warnings that match the arguments,
and the remaining warnings are re-emitted and therefore caught by
`recwarn` fixture.  To provide compatibility with both versions of
pytest, clear the recorded warnings immediately after `pytest.warns()`.

Fixes pydata#8681

xarray/tests/test_groupby.py

@@ -67,6 +67,8 @@ def test_groupby_dims_property(dataset, recwarn) -> None:
     with pytest.warns(UserWarning, match="The `squeeze` kwarg"):
         assert dataset.groupby("x").dims == dataset.isel(x=1).dims
         assert dataset.groupby("y").dims == dataset.isel(y=1).dims
+    # in pytest-8, pytest.warns() no longer clears all warnings
+    recwarn.clear()
 
     # when squeeze=False, no warning should be raised
     assert tuple(dataset.groupby("x", squeeze=False).dims) == tuple(