The XDG Base Directory Specification uses the XDG_CACHE_HOME XDG_CACHE_HOME, not XDG_CACHE_DIR.
Fix test failure on recent Ubuntu Trusty versions without weakening the test itself.
(vila) Fix test failure on recent trusty kernels (the failure can't be
 reproduced in a trusty chroot with a saucy kernel). (Vincent Ladeuil)
(vila) The XDG Base Directory Specification uses the XDG_CACHE_HOME,
 not XDG_CACHE_DIR. (Andrew Starr-Bochicchio)