Fix argument splitting in compiletest

[jieyouxu]

We should fix how arguments are split in compiletest, this involves multiple places:

• The custom diff tool
• //@ compile-flags and such

They usually involve converting from a user-provided escaped string to Command::new() and Command::arg and such, but yeah. Command arg splitting in compiletest is currently very naive and usually just splits on whitespace or is otherwise very basic, we should probably just use a proper library to handle the splitting...

[ChrisDenton]

we should probably just use a proper library to handle the splitting...

Definitely! I believe shlex would be a reasonable choice.