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

rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,d0f/go-zh,d0f/go-zh,bryanxu/go-zh,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,rflanagan/reginaldflanagan-project1,webfd/go-zh,scirelli/scirelli-go,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,scirelli/scirelli-go,mhennings/marcohennings-go,mhennings/marcohennings-go,rdp/rogerpack2005-golang,webfd/go-zh,rdp/rogerpack2005-golang,mhennings/marcohennings-go,d0f/go-zh,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,d0f/go-zh,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,d0f/go-zh,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,webfd/go-zh,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,mhennings/marcohennings-go,webfd/go-zh,bryanxu/go-zh