Update combiner tests to Mega and remove Ultra instantiation

[codygunton]

Originally, we instantiated Protogalaxy over the Ultra arithmetization because 1) Goblin wasn't done; 2) we wanted to run a side-by-side comparison between Protogalaxy IVC and Goblinified Vanilla IVC. It makes sense now to stop instantiating Protogalaxy for Ultra arithmetization. The lone obstruction to doing this is the hard-coded combiner test, which fails if we switch directory to the Mega arithmetization.