os/user: enable tests on all supported platforms
os/user: enable tests on all supported platforms

All of the currently supported platforms have a working user
implementation and do not use stubs. As a result, enable the tests
on all platforms rather than whitelisting.

R=golang-dev, dave, iant
CC=golang-dev
https://codereview.appspot.com/14454044

Committer: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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