test(bigquery): add tests for concatenating categorical columns

[plamut]

Closes #8044.

This PR adds tests for concatenating multiple-page results that contain categorical column data (both with or without pyarrow available).

It appears that the code already works, provided that a correct explicit categorical dtype is passed to to_dataframe() method.

If dtypes are not specified, a default pandas concatenating behavior results in categorical columns ending up with the dtype "object", but that's already explained in the dtypes parameter's docstring, meaning that the code behaves as advertised.

PR checklist

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[plamut]

@tswast Friendly ping. :)

This PR is probably something than can be reviewed and closed quickly (read: before the repo split tomorrow).

[tswast]

Thanks!

[tswast]

[no action required] I suspect there will be users who don't necessarily know the categories ahead of time, though this could trivially be done with a group by / distinct query. If we had a support cookbook, I'd say we should add this info there.