We recently created various "fixture" classes to assist with system-level testing: LogFixture, ClusterFixture and ClientFixture. Each handles the tedious work of setting up the conditions to run certain kinds of tests.
In the same way, we need an OperatorFixture to set up the low-level bits and pieces needed for operator-level, and sub-operator-level unit testing.
The DrillConfig is used by both the system-level and operator-level fixtures. So, pull the config-setup tasks our of (cluster) FixtureBuilder (should rename) and into a new ConfigBuilder. Leave the existing methods in FixtureBuilder, but modify them to be wrappers around the new config builder.