Update requirements.txt

[Inserian]

Added requirement dependencies for tests folder.

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[mhucka]

For the updates in PR #913, we've had to change how the requirements are handled. This PR is correct that the noted requirements need to be added somewhere, but due to the changes in PR #913, requirements.txt is no longer the place to add them. Consequently, I don't see much choice but to close this PR.

Thank you for your contribution nonetheless! It did help to draw attention to the matter.