In mocha >= 7.1.0, exceptions thrown in test hooks (eg. a `beforeEach`
hook) cause the tests to hang without printing any useful exception
details.

From my notes in a Slack investion:

> The karma-mocha package is responsible for running mocha by calling
> mocha.setup() before the test modules are evaluated and then mocha.run()
> after they are evaluated to actually run the tests, as well as listening
> for test events from Mocha’s “runner” and reporting them back to Karma.
> When the beforeEach hook fails, the runner emits a “fail” event which
> karma-mocha receives. karma-mocha then attempts to emit a “test end”
> event and that ends up triggering an exception in a call to
> test.retriedTest() in code which was added in
> https://github.com/mochajs/mocha/pull/4181/files.

Until this issue is resolved, pin mocha to a version that doesn't
trigger this problem.