objectcache: Disable RedisBagOStuff constructor in unit test

objectcache: Disable RedisBagOStuff constructor in unit test

Follows-up c4e698dc2. The tests are failing on Travis CI because
php5-redis isn't installed by default. These tests shouldn't need
that to be installed (and even then, we should skip gracefully as
it is an optional dependency).

> RedisBagOStuffTest::testUnserialize with data set #0
> Exception: RedisConnectionPool requires a Redis client library.
> ./mediawiki/includes/clientpool/RedisConnectionPool.php:86

Fixed by using a mock and disabling the original constructor.

Change-Id: Icced7c30a75516c2118489ad29eac2aa5cff80ad