refactor Retry in tests/cmd/helper.go

[Cryptophobia]

From @vdice on June 24, 2016 18:5

Refactor Retry to accept timeout of type time.Duration as opposed to type int, potentially enabling passing expected exit code was well. Alternatively, in deis/workflow-e2e#237 adds RetryUntilResult which does both, so Retry might be able to just invoke this function for most of its logic...

Copied from original issue: deis/workflow-e2e#240

[Cryptophobia]

From @mboersma on June 28, 2016 22:31

I wonder if https://github.com/codeship/go-retro could simplify any of this code...