This test broke once we began to (properly) push P onto the stack.
However, I'm not _clear_ on whether we should push P before or after we
set the INTERRUPT bit... We'll go with things as-is for now.
Criterion's init and fini config options allow us to register setup and
teardown functions. We now use this to register the common cpu variable
for the mos6502 tests, albeit through the use of a global variable.
Doing so necessitated that each of the different test files have their
own suites, due to some implementation details in Criterion, but this is
No Big Deal.
