rethrow.wast test can be passed by ignoring labelidx

[keithw]

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.