Make it possible to step-wisely run the last failed tests

[pelme]

If I run a lot of tests like

$ pytest
... lots of output
4 failed, 2871 passed in 37.32 seconds

it would be nice if stepwise noted the 4 failures and made it possible to just start with these:

$ pytest --sw  # Run the 4 last failures

I.e. stepwise could record failures from any test run (not just with --sw) and just re-run those until all tests pass.