Increasing test coverage for collections

[jchadwick]

Another set of tests to increase coverage for core functionality

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@jchadwick, thanks for your PR! By analyzing the history of the files in this pull request, we identified @egamma to be a potential reviewer.

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[jchadwick]

As mentioned in PR #13792:

I'm interested and eager to write more unit tests. Please let me know if there are any areas that you would prefer that I focus on (right now I'm just looking at anything called "core").

Also, let me know how to submit the PRs - one test fixture per PR, a bunch at a time, or...?

Happy to help out. Just let me know what you need.

CC @alexandrudima

[jrieken]

Thanks