Improve some of the tests parameters to make them more forgiving.
Set flake8's line width to 88. 88 is black's default and applying black as suggested in the README could break the flake8 test.
Allows cells to take up to 600 sec. Some of the cells are close to the limit and sometime fails.
Have the tests for the broken links only stop the job when a link in a modified file in broken. The check is not stable and sometime fails for links that work manually.
Have v4 and v5's jobs be independent. Here again a random failure in v4 should not block the tests for a new notebook in v5.
Improve some of the tests parameters to make them more forgiving.