os: Fix test to work on Solaris.
os: Fix test to work on Solaris.

On Solaris /bin is a symlink to /usr/bin, so running "pwd" in
the directory "/bin" prints out "/usr/bin".

R=rsc, r, bradfitz
CC=golang-dev
http://codereview.appspot.com/4559043

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