Provide an option for NOT auto-starting the runner

Currently this addon kicks off the test runner after waiting for a while (an attempt to address a chrome sourcemap bug that may or may not still exist)

ember-mocha/vendor/ember-mocha/test-loader.js

Lines 35 to 39 in 13be23a

    
           setTimeout(function() { 
        
             TestLoader.load(); 
        
             mocha.run(); 
        
           }, 250);

Moving away from this, and toward an "explicit start" model like what's done in ember-qunit would provide us with several improvements

Get rid of the setTimeout hack. Developers can call start() whenever appropriate
Provide an opportunity to do things AFTER tests complete loading but BEFORE they start running (currently there's no reasonable opportunity for userland code to run between these two events!)
Provide userland access to the return value of mocha.run() (a Runner instance) such as subscribing to its events
Pass mocha.run a function to invoke when the run is complete

While I think that there's ample reason to make this the default way to use ember-mocha, we should at least be able to easily offer an opt-in path for now, without disturbing the public API

related: #208

Hacky workaround:

vendor/mocha-patch.js

(function() {
  mocha.originalRun = mocha.run;
  mocha.run = function () {};
}());

ember-cli-build.js

app.import('vendor/mocha-patch.js', {
  type: 'test'
});

tests/test-helper.js

import Application from '../app';
import config from '../config/environment';
import { setApplication } from '@ember/test-helpers';

setApplication(Application.create(config.APP));

setTimeout(function() {
  /** @type {mocha.Runner} */
  const runner = mocha.originalRun();
  
  // now we can capture events!
  runner.on('suite end', function (evt) {
    console.log(evt);
  })
}, 400); // < required due to existing 250ms wait in test-loader + some other stuff

@mike-north Did this recently cause you trouble? I have a flaky test that is throwing at mocha.run now where it was previously warning.

@rondale-sc I did not post this in response to something breaking -- I'm just hoping for better consistency between this addon and ember-qunit.

We need to work to push forward the same declarative start() system that exists in ember-qunit. It will likely be "another major bump", but the required app changes are pretty small and the reduction in complexity seems definitely worth it...

	setTimeout(function() {
	TestLoader.load();

	mocha.run();
	}, 250);