Tests aren't cleaning up the resorces

[XVincentX]

Our test suite has multiple parts where async cleanup resources aren't waited for its completion. This introduces flakyness as well as hardening the things when you want to DRY the things.

Example: close method of express applications are async, and they accept a done parameter.

[XVincentX]

With mocha 4.0 this is now kind of a priority. Mocha now does not exit anymore in case there are pending handles (such as sockets, file descriptors and shit like that).

Therefore, we might really need to clean this up, one per time.