Split LayersTest.testDeployedModules in two

Follow up on WFCORE-6466 by splitting the testDeployedModules method in two. The check for 'unused' layers and 'unreferenced' layers are orthogonal and it's clearer for people trying to understand what this is about if we separate them.

The 'unreferenced' test is also treating all 'unused' modules as 'unreferenced' which isn't correct. The two concepts are distinct, and combining them just saved a bit of test config at the cost of correct testing.