Add proxy tests for MFEM integration

[nbeams]

This PR adds two new solver tests that rely on wrapper classes meant to simulate the ones used for the Ginkgo integration in MFEM, to hopefully catch if a proposed Ginkgo change would break the integration with MFEM (or any other library that uses a similar integration approach).

For the purposes of these tests, the wrapped vector type is the Ginkgo MultiVector, since it is a non-Dense type that has the functionality needed from the wrapped type (namely, scale/add and understanding of handling device or host data). In order to handle all the scenarios in the test suite, these proxy wrapper classes have a few notable differences from the actual MFEM wrappers: the wrappers here have a 2D size and understanding of stride (MFEM Vectors do not), and the OperatorWrapper has to be able to handle mixed precision application.

Besides the wrapper classes, the MFEM integration relies on being able to change factory parameters with code like

// Update this object's solver_factory for generating concrete solvers
auto current_params = gko::as<typename SolverType::Factory>
                        (solver_factory)->get_parameters();
this->solver_factory = current_params.with_criteria(this->stop_factory)
                         .on(this->executor);

in order to update the relative tolerance, maximum iterations, etc. I'm not sure if this needs a specific test (or is maybe already covered by a test?).

[yhmtsai]

for update criterion, we have the corresponding test in the core/test/solver/<solver>.

[nbeams]

I tried adding a new test in core/test/solver/cg.cpp for the way we use with_criteria in the MFEM wrappers.