added test and added possible correction for sbc

[rijojohn85]

Hi,
This is my first time submitting a pull request, so please excuse any errors.
Firstly, thank you for making this guide. It's been an amazing learning journey and I've learnt a lot.

I had written tests for all the opcodes for chapter 3.3 and I thought it would be useful to add to the repo for future users.
While testing with your code, I noticed that tests for SBC was failing. On further inspectioin, I noticed that there MIGHT be a error with your SBC code.

In the book you had mentioned

After ADC is implemented, implementing SBC becomes trivial as A - B = A + (-B). And -B = !B + 1

but I'm not sure if you implemented it in your code:
fn sbc(&mut self, mode: &AddressingMode) { let addr = self.get_operand_address(&mode); let data = self.mem_read(addr); self.add_to_register_a(((data as i8).wrapping_neg().wrapping_sub(1)) as u8); }
My tests were always failing by 1. I tried 10-10. It should have been 0, but I'm getting 255 with your code. I changed the code a bit and I was getting the code to pass the tests. It's below.

fn sbc(&mut self, mode: &AddressingMode) { let addr = self.get_operand_address(mode); let data = self.mem_read(addr); self.add_to_register_a(!data + 1); }

Let me know if this is correct, else I'll have a look again and. But I think the tests will be useful otherwise as well.