Use Jest as a Test runner

[Sarah-Seo]

We need a custom test runner for compiled code by webpack on electron.

Currently, I wrote the codes from scratch. It uses webpack HMR so runs test very fast.
But, the test runner has lack of functionality. It should be improved more.

Anyway, I'm going to mimic the api of Jest. If you have any good idea, please tell me. We should discuss more.

[Sarah-Seo]

I changed my mind. Jest will be used instead of the current custom test runner.

• Jest would be enough to test the current coverage.
• Not enough resources to maintain own test runner.

If some api is needed to hit electron api, the test should be conducted on Spectron.