Add AMG performance tests #361
Description
Description of feature
The AMG tests only check if the flow and adjoint solvers converge. We should add tests to make sure the rates of convergence follow the expected trends when using different solver options.
There are some challenges to doing this. Our current regression testing infrastructure is not well-suited to testing performance. In addition, differences in performance might only show up on larger cases.
See #359 for some related discussion