os: add tests for operations on nil *File methods
os: add tests for operations on nil *File methods

R=shawn.p.smith, gobot, r
CC=golang-codereviews
https://codereview.appspot.com/46820043

pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme