Enable testing in MacOS via Github Actions

9fef761 implemented testing for Windows and Ubuntu via Github Actions.

However, for GitHub's MacOS image does not come with Mercurial installed, so we cannot run tests on it the same way.

This issue is for enabling testing in MacOS. One possible way is by installing Mercurial via pip.

I install stuff on Mac using homebrew, but pip is probably the way to go here because installation would be much faster and ensures hg will use the system python. Catalina ships with both 2.7 and 3, and the python command runs 2.7. You gotta run python3 and pip3 to run Python 3, and then we'd have to tell VSCode to use those python executables as well. Everything should work fine in 2.7 anyway.