[test] Check that backwards branches trim the stack

[Sainan]

Wasn't really sure where to put it or what to call it, but I hope this is fine.

Closes #2080

[tlively]

test/core/br_if.wast would probably be a better location since this is part of the behavior of br_if more than loops.

[Sainan]

Alright, moved it there.

[rossberg]

We have many tests that test this property of br_if (e.g., various of the as-* ones in this file). But they all target block by default. So I would argue this is more specific to loops, because it tests back edges in particular. We can put it into br_if.wast, but then I would expect equivalent additions to the other br* files.

It would be good to also have a variant that passes some loop parameters.

[Sainan]

I did try writing this using br initially but I kept producing non-terminating code. I do suspect br_if is unavoidable for this test.

[rossberg]

You'll need a separate branch for terminating the loop. It's probably not worht it, but then I think it makes more sense to keep the test in loop.wast (sorry for the churn :) ).

[Sainan]

Okay, pointed the branch at a1b91d4 again ☺️