Make sure test case depends on existing bucket.

Motivation
----------
The ConnectionTest did not depend on the general "ClusterDependentTest"
which made it subject to a potential non-existing bucket, leading to
false-positive assertion errors.

Modifications
-------------
Making it extend ClusterDependentTest.

Result
------
No more failing tests because the bucket doesn't exist by accident.

Change-Id: I4b25876114c78621ba392c1dbdaee64e5d16393d
Reviewed-on: http://review.couchbase.org/42320
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Simon Baslé <simon@couchbase.com>