Split test fixtures in test_filter_metadata.py

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Minor issue that cropped up in the review of #164.

In `test_filter_metadata.py`, there are the following 3 functions:
* `filter_one_column`
* `filter_multiple_and`
* `filter_multiple_or`

These functions are essentially test fixtures, but, instead of being treated as fixtures, they are just functions that are combined into one fixture called `test_filter_metadata`. As a result of having this "super-fixture", we can't easily detect which of the 3 functions above actually failed.

This "super-fixture" should be split into 3 different fixtures to test these functions independently and simplify failure tracking.

Additionally, per the discussion in #164, these functions are (for some reason) also used in:
* `thicket/tests/test_concat_thickets.py`
* `thicket/tests/test_groupby.py`

As a result, these functions should be moved into some "testing utils" module. Then, `test_filter_metadata.py` should import the functions and use them in the 3 new fixtures, and the other two modules (listed above) should import the functions and use them as they currently are.

Start by reading test_filter_metadata.py and the usages in thicket/tests/test_concat_thickets.py and thicket/tests/test_groupby.py. Move the three named functions into a testing utils module, import them where needed, and split test_filter_metadata into three independent fixtures. Run the affected tests and confirm each fixture reports failures separately.

Written by the indexing model from the issue text.