Support inequality validation for `post` in tests

Right now, we support validation for an `Account` in `post` to have its storage set as some value, or its balance be some value. For values that alter the starting state but is maybe a hash or some undetermined value, or even for tests where we may just want to make sure this value is _different_, it would be nice to have some way to validate this. It has more recently come up in a couple of different contexts:

- BAL validation [here](https://github.com/ethereum/execution-specs/pull/2113#issuecomment-3871478893)
- Benchmark testing for BALs [here](https://github.com/jochem-brouwer/execution-specs/pull/1#discussion_r2808563885)

It would be nice to be able to support that some starting value has _changed_ at `post`... or even perhaps that it doesn't equal some value. It's a bit slippery but with great power comes great responsibility (🕷️) and I think we could distinguish well enough when this is warranted and when it isn't. I can see the argument to not have this at all as well though.