Add more tests

[jeremydavis519]

The current set of tests is extremely sparse. It touches every part of the parser, but in some places, only slightly. In particular, we need to test weird combinations of scoping, multiple nested calls to rusty_asm!, and multiple asm blocks with bridge variables defined between them. There are a lot of cases where we should make sure the expanded code doesn't compile.

[jeremydavis519]

I've added some more tests and covered most lines, but we could always use more. And they're currently all x86-only, although that shouldn't really be a problem, since rusty-asm doesn't interact with the instruction set at all.

Also, some of them aren't very effective tests. It would be useful to confirm that the macro produces exactly the expected code--for instance, that the inputs, outputs, and clobbers in each asm! invocation are exactly what's needed.