Add compile-pass header from Rust's compiletest

[phansch]

Rust's compiletest supports a // compile-pass header, which is the same as // run-pass, but without executing the resulting test binary.

It would be nice if compiletest-rs supported this header, too.

[phansch]

Actually, this is not quite true. It was just renamed in rustc to compile-pass and was not renamed here. Here it's called must-compile-successfully. However, it looks like there is still some part of the implementation missing here.