The execute function should just work from the PC register. It might
seem to be easier to test by passing an arbitrary opcode into the
function, but because so much of the chip's execution is
context-sensitive (that is, it expects PC to be pointing at the opcode,
to have its operand in front of it, etc.), passing an arbitrary opcode
is not really reflective of what needs to be in place for the function
to work correctly.
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.
