testing: use the official gocheck package on GitHub

https://github.com/go-check/check
gocheck vs. check (default) vs. `import . "gopkg.in/check.v1"`?

FYI, the [gocheck documentation](http://labix.org/gocheck) use `import .`, and so does [amz.v1](https://github.com/go-amz/amz/blob/v1/s3/s3_test.go#L10).