Replacing custom testing helpers with testify, where possible

[paulbellamy]

Closes #358

[rade]

@inercia @awh would appreciate you taking a look at this.

I have suggested to @paulbellamy that we should file PRs against testify for adding the two helpers we are currently left with.

[inercia]

Looks good to me. It's great to use someone's else library for this...

[inercia]

Regarding testify missing the stacktraces, it would be great if we could be more general and call any user-provided callback when Fatalf is invoked. This callback could be a function for dumping the stack trace, but it could also be a function for dumping all the goroutines stacks, stop the debugger, etc...