Use include_str! in test

[rootkc]

I use Rust skeptic for managing my snippets for SOME. I was creating a snippet like this:

fn main() {
    // The contents of file.txt is "Hello world!"
    let str_from_file = include_str!("file.txt");
    assert_eq!(str_from_file, "Hello world!");
}

And I Couldn't get it to work because file.txt wasn't available at compile time.

How would I add external files that are supposed to be compiled into the tests? This is super useful for Internationalization like Fluent and Template libraries like Tera.

[djc]

See #140.