ui_test panics when Rust ICEs

[RalfJung]

This line here can panic:

miri/ui_test/src/rustc_stderr.rs

Line 85 in 4b100a1

let msg = Message { level: self.level.parse().unwrap(), message: self.message };

thread '<unnamed>' panicked at 'called `Result::unwrap()` on an `Err` value: "unknown level `error: internal compiler error`"', ui_test/src/rustc_stderr.rs:85:55

Obviously Rust should never ICE, but when it does, ui-test shouldn't panic either.