Introduce testutil package

[lucapette]

In other Go projects, I'm enjoying having a package pkg/testutil with test helpers. It helps polishing the testing code and encourages more testing (which is hardly a bad thing). To get it started we could introduce a testutil.Diff(a, b interface{}) err and use it wherever we make use of github.com/kr/pretty. Right now we use it in the integration tests only but if we have it in a package then we can use it in the unit tests too.