Make TestReaddirWalk umask independant.

Fixes bradfitz/docker#6

Extract a helper function and walk both client and server trees.

Docker-DCO-1.1-Signed-off-by: Paul Nasrat <pnasrat@gmail.com> (github: pnasrat)
Merge pull request #10 from pnasrat/fix-readdir-walk-test

Fix readdir walk test