stepwise: also remember last successful test for continuation

[blueyed]

I think it would be useful if the stepwise plugin would also take into account successful tests.

A use case is running all tests, Ctrl-C'ing them (KeyboardInterrupt), but then wanting to continue from where they stopped. For this session.exitstatus = 2 (interrupted) could be used in pytest_sessionfinish.

[graingert]

@blueyed I think stepwise needs to remember all tests and their state, "passed", "failed", "skipped", "pending"

This way it can handle:

• the failed test being renamed
• new tests added "in front" of the step-failing test
• xdist tests failing "in front" of the step-failing test, and the step failing test failing after a pending test
• pytest randomly

[graingert]

"--lf" and "--sw" should share the same database