fs_test.go doesn't finish if Docker's code is placed under a directory
which has symlinks between / and the directory.
For example, the below doesn't finish before the change.
/home -> usr/home
FollowSymlinkInScope("/home/bob/foo/bar", "/home/bob/foo")
Docker-DCO-1.1-Signed-off-by: Kato Kazuyoshi <kato.kazuyoshi@gmail.com> (github: kzys)