Fuzzer: Fix Merge fuzzing when input must be ignored

[kripken]

I saw a failure where the second module hit a VM limit, so it should be
ignored, but we weren't, and ended up thinking it behaved differently
after merge.

[tlively]

Alternatively, we could check that the merged output is ignored whenever either of the original modules' output is ignored (or even check more specifically for matching errors).

LGTM if you prefer to keep this simpler, though.

[kripken]

Hmm, I think that isn't guaranteed. We may ignore due to a host limitation, and wasm-merge does some cleanups in the merged module (remove-unused-module-elements). That might make it just pass under a host limit that existed in one of the inputs.

[tlively]

Makes sense.