Testing polars support for joiner, fuzzy_join

[Théo Jolivet]

Partially closing #769 for test_fuzzy_join.py and test_joiner.py

Added XFAIL_POLARS = ["joiner", "fuzzy_join"] in skrub.dataframe.tests.test_polars since joiner and fuzzy_join don't work with Polars inputs yet

The issue was that there is no reset_index in Polars