Fix engine tests on systems where temp directories are symlinked.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Document Darwin test failure in a FIXME

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Merge pull request #4167 from shykes/fix-tests-mac

Fix or document broken tests on mac