Replace Karma (Deprecated) as the JS Test Runner

### Product

axe-core

### Product Version

4.9.0

### Latest Version

- [X] I have tested the issue with the latest version of the product

### Issue Description

#### Expectation

Test runner operates on a reasonably standard library that is well maintained and documented. Likely mocha, as that is what is used for other tests.

#### Actual

`axe-core` test running operates on a deprecated framework called [Karma](https://github.com/karma-runner/karma/)

> Karma is deprecated and is not accepting new features or general bug fixes.

#### How to Reproduce

`npm run test:unit` or any of it's sibling scripts, a browser appears running karma.

#### Additional context

This is very likely _not_ a security concern, as far as I can see we never import Karma in such a way to introduce a problem and it's only used to run tests.