Fix mocha version and unit tests

[kevinrambaud]

As mentioned in comment https://github.com/koajs/examples/pull/114#issuecomment-359583756, builds are failing due to --no-exit default flag since mocha 4.0.0

To fix this issue, I did the following changes :

• removed supertest duplicate dependency
• fixed mocha version
• updated all test files to empty event loop after running test (the koa app instance were still running)

[kevinrambaud]

Do I need to do any action to make it mergeable by you guys ?

[dead-horse]

thanks!