[wip] Make mocha work with ember-exam v3

[step2yeung]

Problem:
There is no test-loader in the latest release of ember-mocha. Hence users are seeing the following error in the browser console error:

Uncaught Error: Unable to find a suitable test loader. You should ensure that 
one of `ember-qunit` or `ember-mocha` are added as dependencies. at 
http://localhost:7357/assets/test-support.js, line 51528

reference: #238

Fix:

• Change getTestLoader() to search for ember-mocha or ember-qunit from require
• Change ember-exam-mocha-test-loader to extend from ember-cli-test-loader instead of ember-mocha/test-loader which has not been released yet

Other work:

• move loadTests back to load.js

Tests:

• Test with Ghost-admin
• Add test that tests all ember-exam features against mocha

[step2yeung]

This won't work since tests are being ran twice, one by calling start() here, another time by autostart from ember-mocha. The only way this will work is to re-introduce monkey-patching of ember-cli-test-loader, which we moved away from in v3.
Mitigation is to use < v3.0 of ember-exam