`VmInstanceRotMock` isn't a mock impl

[flihp]

This type started out as a mock impl of the VmInstanceRot trait. Turns out, the mock impl that matters is the one implementing the dice_verifier::Attest trait. This is passed to VmInstanceRotMock when it's constructed, but the internals of the VmInstanceRotMock are unaffected. This type needs a different / less inaccurate name, and should be moved out of the mock module.

ref: oxidecomputer/propolis#1076 (comment)