tests: fix mocha test runner and mocks on windows

[connorjclark]

Fixes #14171

I'm using my fork from my PR to quibble: testdouble/quibble#71

Also fix a couple things related to windows paths (done in #14464).

[connorjclark]

Note: the quibble PR fixes things related to mocking ES modules. We only do a handful of mocks for ES modules in master right now, so this isn't a great test of the quibble PR. I merged this PR (14202) into the core ESM PR (#14182) and get a handful of test failures related to ESM mocks, so there's still more to do there. But it seems at least the basic usage we do atm in master is working. It also may be that quibble is fine now, and the bug exists in our code re: proper usage of import.

Mostly writing this as a reminder to myself to look into this Monday.

[connorjclark]

I believe I fixed all of the windows issues. The remaining test failures remind me of what #14216 is meant to avoid, so let's wait for that to land first.

[connorjclark]

Passing in CI, and confirmed to work locally. At least, unit-cli does.

Some core tests (like pptr scenarios) hang on the after hook. Something to look into one day, but not high priority.