time: add tests for Duration.Nanoseconds, Duration.Minutes, and Duration.Hours
time: add tests for Duration.Nanoseconds, Duration.Minutes, and Duration.Hours

R=golang-codereviews, rsc, dave
CC=golang-codereviews
https://codereview.appspot.com/42440043

Committer: Dave Cheney <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@cheney.net>

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang