start adding some more tests for version updates

[wolfv]

Continuing the implementation here, but potentially moving it into cf-scripts afterwards.

I've added some interesting test cases with one that is kinda hard to fix: the polars test.

[wolfv]

OK, actually the polars test is also marked as xfail:

    pytest.param(
        "polars_mixed_selectors",
        "1.1.0",
        marks=pytest.mark.xfail(reason=VARIANT_SOURCES_NOT_IMPLEMENTED),
    ),
    pytest.param(
        "polars_name_selectors",
        "1.1.0",
        marks=pytest.mark.xfail(reason=VARIANT_SOURCES_NOT_IMPLEMENTED),
    ),
    pytest.param(
        "polars_variant_selectors",
        "1.1.0",
        marks=pytest.mark.xfail(reason=VARIANT_SOURCES_NOT_IMPLEMENTED),
    ),

[Hofer-Julian]

Nits, apart from that looks good to me