os: improve Readdir and Readdirnames test coverage
os: improve Readdir and Readdirnames test coverage

R=golang-codereviews, dave
CC=golang-codereviews
https://codereview.appspot.com/46450043

Committer: Dave Cheney <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@cheney.net>

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang