Add initial client unit test.

[rwjblue]

Adds basic client unit testing setup using ember-mocha.

There are many example blueprints for unit tests using ember-cli-mocha here. These are written for Ember CLI, but apply nearly the same here.

My hope is that this is just the beginning of many unit tests for the admin client.

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Running testem in the root of the project brings up the testem runner:

Navigating to http://localhost:7357/core/test/client/index.html then shows the browser tests:

Addresses #2990.

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p.s. Sorry it took me so long...

[jaswilli]

Awesome. 🎉

[ErisDS]

So much epic 🎊 🎈 🍰

@rwjblue THANK YOU soooooooo much for taking the time to do this 😁
@jaswilli I totally agree - but I think this is going to be the trigger for a few restructuring changes to the tests and we can do that separately :)