Increase test coverage for collate

[d33bs]

In working through #437 via #439 I noticed that collate has collate has much lower test coverage than other components within pycytominer. We could increase this coverage with some further tests, possibly while also addressing #315 and #394 .

[gwaybio]

Yes, we could, in theory increase coverage. See #160 (review) for our initial discussion of code coverage for collate

Given the march toward 2.0, which won't support collate, I'm not sure if it's a wise time investment to increase coverage at this time. Also, there have been no complaints about this functionality not working to my knowledge.

Also, we deprecated cytominer-database last week. https://github.com/cytomining/cytominer-database