pkg/mount/mounter_linux_test: nits
pkg/mount/mounter_linux_test: nits

Fix test error messages:
 - add missing commas;
 - fix a typo (failed -> field)
 - allow for clear distinction between mount
   and vfs (per-superblock) options.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

containers/storage,containers/storage,rhatdan/storage,rhatdan/storage,containers/storage,rhatdan/storage