Add mocker.resetall()

[mathrick]

Having a mocker.resetall() method, similar to mocker.stopall(), would be very handy. Most of the time if I mock things, it's because I'm testing a side-effecting function which cannot be blackboxed effectively, and will want to walk through multiple code paths while checking that various callees were/were not called as appropriate. This commonly involves resetting the mocks, so not having to reset them individually would greatly increase the usefulness of mocker.

[nicoddemus]

Thanks for the feedback @mathrick. I'll try to get to this ASAP, but meanwhile a PR would be welcome. 😄

[mathrick]

I gave it a shot, but it turns out to be more complex than stopall(), since unlike .stop(), .reset_mock() isn't defined on the mock._patch class, and the multiple levels of indirection are a pain to figure out. So for now I consider myself defeated.