Rewrite compiletest to use `Command` liberally

[alexcrichton]

Believe it or not but the compiletest project predates the usage of Command in the standard library. We currently pass through tons of variables in tons of places, eventually culminating in creating a Command at the very end. We should instead thread through a Command everywhere to make argument passing much nicer!

That way we also don't need functions with seven arguments!

[euclio]

I'd like to work on this.

[alexcrichton]

Awesome thanks @euclio! If you've got any questions please don't hesitate to ask!