refactor(tests): replace pytest.fail with pytest.xfail

Updated the `governance_poll_available` fixture to use `pytest.xfail`
instead of `pytest.fail` for better handling of unavailable poll commands.
This ensures tests are marked as expected failures rather than hard failures.

Author: mkoura

cardano_node_tests/tests/test_governance.py

@@ -36,7 +36,7 @@ class TestPoll:
     @pytest.fixture(scope="class")
     def governance_poll_available(self) -> None:
         if not clusterlib_utils.cli_has("compatible babbage governance create-poll"):
-            pytest.fail(
+            pytest.xfail(
                 "The `cardano-cli compatible babbage governance` poll commands are not available."
             )