test: Try invoking ibis.set_backend only in scopes that use it

May fix https://github.com/vega/altair/actions/runs/11438192791/job/31819285124?pr=3647

tests/vegalite/v5/test_api.py

@@ -31,7 +31,6 @@
     from altair.vegalite.v5.api import _Conditional, _Conditions
     from altair.vegalite.v5.schema._typing import Map
 
-ibis.set_backend("polars")
 
 PANDAS_VERSION = Version(importlib_version("pandas"))
 
@@ -1613,6 +1612,7 @@ def test_polars_with_pandas_nor_pyarrow(monkeypatch: pytest.MonkeyPatch):
     sys.platform == "win32", reason="Timezone database is not installed on Windows"
 )
 def test_ibis_with_date_32():
+    ibis.set_backend("polars")
     df = pl.DataFrame(
         {"a": [1, 2, 3], "b": [date(2020, 1, 1), date(2020, 1, 2), date(2020, 1, 3)]}
     )
@@ -1633,6 +1633,7 @@ def test_ibis_with_date_32():
     sys.platform == "win32", reason="Timezone database is not installed on Windows"
 )
 def test_ibis_with_vegafusion(monkeypatch: pytest.MonkeyPatch):
+    ibis.set_backend("polars")
     df = pl.DataFrame(
         {
             "a": [1, 2, 3],