Fix test import display

This PR should solve the last issue related with #6520 (Go 1.3 bump).

[This change in Go 1.3](https://code.google.com/p/go/source/detail?r=d92b32d188ec18b61690d3c8e74c02e03da66105&name=go1.3) revealed in [progressreader.go](https://github.com/docker/docker/blob/master/utils/progressreader.go#L35) an `io.Reader` ambiguity on EOF.

From the docs of `io.Reader`:
> When Read encounters an error or end-of-file condition after successfully reading n > 0 bytes, it returns the number of bytes read. It may return the (non-nil) error from the same call or return the error (and n == 0) from a subsequent call. An instance of this general case is that a Reader returning a non-zero number of bytes at the end of the input stream may return either err == EOF or err == nil.