pytest-xdist incompatible with generating files

[ConorMacBride]

When pytest is run over multiple processes/machines using pytest-xdist, generating files such as baseline images, baseline hashes, result and diff images, and JSON and HTML summaries may not work as expected. This is due to the tests being run in different directories in different processes, as well as information not being shared between processes which is necessary for generating a full summary or hash library.

However, the result of each test will be reported correctly by pytest, so using xdist should be fine if you are not using these pytest-mpl features.

This should be documented, and a warning should be given if using pytest-xdist. Ideally support for pytest-xdist would be added.