Setup test cases for evaluation of instruction semantics

I'd like to be able to write something like:

```prolog
test(instr_li, [State.regs.x == 7]) :-
run((
li($x, #7)
), State).
```

Start by locating the existing run(...) entry point and the instruction semantics for li($x, #7). Investigate how the shown test(instr_li, [State.regs.x == 7]) form could be registered and evaluated. Done means equivalent instruction tests can be written and the register assertion is checked.

Written by the indexing model from the issue text.