MBC/IEC Test Refactor

[GabrielKS]

The MBC and IEC tests ought to be made more readable and robust. In particular:

• Make tests more robust: right now we need to run with very fine tolerances to make things work, and still it's inconsistent between machines
• Figure out what to put in PSB, and put it there. This is made less straightforward by the fact that every little @testset essentially uses its own system (which is vital to the testing methodology). Maybe a parametric build_system?
• Make the rest of the MBC/IEC test helpers more readable without drastically increasing code duplication

[luke-kiernan]

I'll close this. We now have modular tests of the MBC stuff. The second issue is still relevant, but it belongs in PSI or PSB, not here.