Use go style duration instead of uint to represent millis

The production value will probably be something like 30 seconds which
reads as 30000 in millis, a little awkward. The test value is 500 millis
which doesn't work as a uint representing seconds. Each of these would
be 30s and 500ms respectively, much more readable in our opinion.

Also see http://golang.org/pkg/time/#ParseDuration

[#82968904]
Use assertion for error checking since this is a spec

[#82968904]