Tests README: Add instructions to install testing-related packages

[mathieuboudreau]

Checklist

GitHub

• I've given this PR a concise, self-descriptive, and meaningful title
• I've linked relevant issues in the PR body
• I've applied the relevant labels to this PR
• I've assigned a reviewer

PR contents

• I've consulted ivadomed's internal developer documentation to ensure my contribution is in line with any relevant design decisions
• I've added relevant tests for my contribution
• I've updated the relevant documentation for my changes, including argparse descriptions, docstrings, and ReadTheDocs tutorial pages

Description

In the testing/ README, it was previously assumed that you already had pytest and pytest_console_scripts already installed, but these are not installed along with the IVADOMED requirements. I've added instructions to install them, otherwise users might get an error (I did).

Linked issues

[coveralls]

Pull Request Test Coverage Report for Build 2770497171

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+9.8%) to 78.975%

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Totals
Change from base Build 2770265565:	9.8%
Covered Lines:	4838
Relevant Lines:	6126

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💛 - Coveralls

[dyt811]

Good call. I agree. This was actually because we USED to have a requirement file for those for dev specifically but since a recent PR streamlining the installation process, it has been borked.

[dyt811]

Thanks!

[kousu]

Thanks for running through the install process and catching these problems. These are the sorts of things that fall through the cracks when people write software together. See my comments abovebelow for what I think should be done.

I'm sorry it got lost in my inbox @mathieuboudreau.

[kanishk16]

Thanks @mathieuboudreau as we would have definitely missed this! LGTM apart from a minor suggestion which is continuation to Nick's comment.

[mathieuboudreau]

Thank you all for your reviews and suggestions! Merging!