Add pytest-clarity to test environment to improve error log output

[yubiuser]

We use pytest for testing. While it is very capable, its error output is a mess. Adding pytest-clarity improves on that (see https://github.com/darrenburns/pytest-clarity).

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Note: this PR needs 99f459e to pass the tests

[rdwebdesign]

I really liked the new output, but since the intention is to improve the readability, I think we could use longer lines.

In my opinion, using the default width (80 characters) is creating too many line breaks in the output:

We already use line breaks (/n) where we need (and the github interface already prevents horizontal scrollbars).
We could have lines with 120 characters or any other width, adding the diff-width parameter here:

py.test -vv --diff-width=120 -n auto ./test/tests/

What do you think?

[PromoFaux]

❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️ ❤️

But also, what @rdwebdesign said.

[PromoFaux]

Although it does not seem to make a difference.

Still 80 chars with new lines is better than no new lines at all, so I'll take it!

Before:

After:

[rdwebdesign]

Well... the --diff-width parameter didn't work as I expected, but I was able to change the width using the COLUMNS environment variable:

COLUMNS=150 py.test -vv -n auto ./test/tests/