Fixed backup unittests to use mocked cmd

The backup unittests were using the actual cmd from the backup
implementation (eg. innobackupex) instead of the faked unit test
cmd. Fixed this so that calls to innobackupex are no longer being
made when running the unit tests.

Closes-bug: 1290290
Change-Id: I26b77efbf95feb5a32777197c7cba08f7b757c38
Merge "Fixed backup unittests to use mocked cmd"