[test] Check content of error message

[szabgab]

in failure_on_missing_theme_directory

[Rabbival]

I do find it a little offsetting that you check the string literal within the message, meaning that if someone happens to change a single char there in the future the test would fail.
Also, and that one is more of a matter of personal taste, you might get a more informative test (and a slightly cleaner one) if you wrote it like:

match got{
    Err(e) => { assert_eq1(e.to_string(), "Rendering failed"); },
    Ok(content) => { panic!("Expected an error but got an Ok with content: {:?}", content); }
}

[ehuss]

I'm going to close as superseded by #2676, which is able to validate the entire output.