Spurious failures fix for bug-948729.t and bug-948729-mode-script.t

Change I816b36e1d3e6933f92acf19d9be8eeaaa333356e causes sprurious
failures in bug-948729.t and bug-948729-mode-script.t
This seems to be caused by leftover information in /d/backends/
probably in a dot-prefixed directory.

Attempt to fix this by removing /d/backends and other work directories
on cleanup. While there we test and report failure to remove anything.

BUG: 1129939
Change-Id: I0373c7f3c507db8ab509e8fd3aca2a386bea807e
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/9081
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>
Tested-by: Pranith Kumar Karampuri <pkarampu@redhat.com>