Support multiple `#[should_panic]` invocations

[skogseth]

With the current implementation in #151 the #[should_panic] macro can only be applied once, annotating a function with multiple #[should_panic] invocations will lead to a compilation error. We could potentially support invoking this multiple times with different expected messages, and then succeeding if any of the expected messages are contained within the error message. Another option is to follow libtest, which simply ignores subsequent calls to #[should_panic].

[epage]

I feel like there is a clear meaning for multiple ignores when we get #48 (apply each, effectively or-ing them).

If we take the same approach of what I'd expect for #48, applying each, then having multiple becomes mutually exclusive.