Replace mockery

[maurolepore]

Closes #57

This PR is part of the tidyverse developer day. It removes mockery as a dependency but turned out quite messy. Feel free to close it. No hurt feelings :-)

In particular here are potential problems I see:

• The tests may not expose some problems. Many tests are called via a wrapper to test_that() which I didn't fully understand. Some tests seem to fail interactively but don't fail when calling test().
• In R/git-auth.R I messed up the order in which the functions are defined, sorry :-(.
• local() still has some functions. If local() was necessary to support mockery, then removing mockery should result in extracting all functions out of local() but that's not the case.
• R CMD check and test coverage fail on GitHub Actions for Windows and MacOS.

[gaborcsardi]

Thanks for taking a stab at this! It seems like this is more difficult than I anticipated, so I am going to close this now, and may take another look later. Thanks again!