Macro assert_warning

macro_rules! assert_warning {
    ($ctx:expr, $cond: expr $(,)?) => { ... };
    ($ctx:expr, $cond: expr, $msg:expr $(,)?) => { ... };
    ($ctx:expr, $cond: expr, $msg:expr, $($frag:expr),+ $(,)?) => { ... };
}

Check the condition and emit a Diagnostics::warning if it fails.