Tests: Add missing dependency and improve documentation

[amotl]

Dear maintainers of magics-python,

first things first: Thanks a stack for your work on this software.

While trying to investigate a problem with Magics.version on macOS, we discovered some gaps in the documentation how to run the test suite. After figuring it out, we wanted to share it back on behalf of improvements to the documentation.

So, this patch fills in some gaps to educate people how to exactly run the full test suite. Along the lines, it adds the missing pytest-flakes package.

Within the README, it adds a note about setting the MAGPLUS_HOME environment variable, which is helpful in non-standard setup location environments.

With kind regards, Andreas.

[FussyDuck]

All committers have signed the CLA.