Adding test case for symlink causes infinit loop, reproduces: dotcloud#5370

normally symlinks are created as either
ln -s /path/existing /path/new-name
or
cd /path && ln -s ./existing new-name

but one can create it this way
cd /path && ln -s existing new-name

this drives FollowSymlinkInScope into infinite loop

Docker-DCO-1.1-Signed-off-by: Lajos Papp <lajos.papp@sequenceiq.com> (github: lalyos)
Defend against infinite loop when following symlinks

ideally it should never reach it, but there was already multiple issues with infinite loop
at following symlinks. this fixes hanging unit tests

Docker-DCO-1.1-Signed-off-by: Lajos Papp <lajos.papp@sequenceiq.com> (github: lalyos)
Fixes 5370 infinite/maxLoopCount loop for relative symlinks

use path.IsAbs() instead of checking if first char is '/'

Docker-DCO-1.1-Signed-off-by: Lajos Papp <lajos.papp@sequenceiq.com> (github: lalyos)
Merge pull request #5396 from lalyos/5370-infinite-loop-caused-by-symlink

5370 infinite loop caused by symlink