os: make TestMkdirAllWithSymlink more robust
os: make TestMkdirAllWithSymlink more robust

Don't assume the test has a clean environment within /tmp.
Use an actual new tempdir for its tests.

Fixes FreeBSD build failure as seen at:
http://build.golang.org/log/396738676356d7fb6bab6eaf1b97cac820f8a90f

--- FAIL: TestMkdirAllWithSymlink (0.00 seconds)
path_test.go:178:                 Mkdir /tmp/dir: mkdir /tmp/dir: file exists
FAIL
FAIL    os      1.091s

R=golang-dev, minux.ma
CC=golang-dev
http://codereview.appspot.com/6615057

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