fix(evm): check for global failure instead of DSTest flag when reverting snapshots

[Evalir]

Motivation

Closes #3792.

The root issue of these soundness issues comes from storage layout shifts depending on the inheritance order of the test contract. We, right now, check for the _failed flag on DSTest, which, assuming the user has the correct inheritance order, is a valid way to ensure we're keeping track of snapshot failures. However, if it's not correct, the flag will not be at the expected slot, and therefore we'll get unsound behavior.

Solution

What we do instead is now rely on the global failure slot that dstest keeps on the hevm address (keccak256("failed")) to check for failures. This should avoid any unsoundness issues regarding the inheritance order when it comes to snapshots.

[Evalir]

I'm a bit suspicious of the is_global_failure function. While debugging, the "failed" slot should absolutely be set to 1 on repro 3055, but the function is not returning this. However, logging the slot with dstest will show that the slot is indeed 1. Will check if there's something wrong in this func tomorrow

EDIT: Just needed same treatment as the usual way to check contract state—check present value.

[mattsse]

nice find!

needs some docs for the slot

[mattsse]

needs docs that this is keccak("failed")

[Evalir]

done! a75f9fb

a note: this is actually only "failed" in bytes and padded—not keccak("failed")