Clean up callbacks from old engines in engine_testapp

If we leave old callbacks around then calling one of them can
trigger a callback on a NULL engine in test cases where we
reload the engine in the middle of the test.

Change-Id: I519c6dcd3647420774c30cf0444bc352046cae71
Reviewed-on: http://review.couchbase.org/36654
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>