Add convergence test for indefinite integrals

We should add spatial convergence tests to our indefinite integrals. Cross ref discussion: https://github.com/CliMA/ClimaCore.jl/pull/1504#issuecomment-1773070518. Doing this requires collecting the errors and, at the moment this involves 2 loops already, maintaining a 3-tuple dictionary seems like a bit overkill (we need to collect the errors to compute the convergence rate). Maybe there's a simple way we can do this, but it requires a bit of thought. I've already (offline) confirmed that the operator is 2nd order accurate. Also, the existing tests in ClimaAtmos fail with small MSE error changes, so I think this is closely approximating `SciMLBase.solve`.

Start with the linked pull-request discussion and the existing indefinite-integral tests. Determine how to collect errors across spatial resolutions without an unnecessary 3-tuple dictionary, then use those errors to calculate the convergence rate. Done means the tests verify the expected second-order spatial accuracy while accounting for the existing ClimaAtmos and SciMLBase.solve behavior.

Written by the indexing model from the issue text.