Convert tests to es modules

This may turn out to need to be split into more than one ticket, but putting a general ticket in for now. We need to babel our unit tests so we can update them to use es modules (and es6). Once we do that, we can fix some of the unit tests that we commented out or skipped due to no way to mock functions.

Edit by Wilco:
We have been disabling a number of tests, as part of converting to ES modules, those tests need to be re-enabled:

- [x] https://github.com/dequelabs/axe-core/pull/2167
- [x] https://github.com/dequelabs/axe-core/pull/2159