rethrow.wast test can be passed by ignoring labelidx

It's possible to pass the current rethrow test (https://github.com/WebAssembly/exception-handling/blob/main/test/core/rethrow.wast) with an implementation that ignores the `labelidx` and always rethrows the innermost (or outermost, or an arbitrary...) caught exception.

It would be good to test that the implementation rethrows the correct caught exception.