test(vm): Run multivm tests with shadowing (#3137)

## What ❔

Runs shared unit tests in the `multivm` crate for the shadowed VM.

## Why ❔

Allows to cheaply check VM divergences.

## Checklist

- [x] PR title corresponds to the body of PR (we generate changelog
entries from PRs).
- [x] Tests for the changes have been added / updated.
- [x] Documentation comments have been added / updated.
- [x] Code has been formatted via `zkstack dev fmt` and `zkstack dev
lint`.

core/lib/multivm/README.md

@@ -14,5 +14,7 @@ If you want to add unit tests for the VM wrapper, consider the following:
 - Whenever possible, make tests reusable; declare test logic in the [`testonly`](src/versions/testonly/mod.rs) module,
   and then instantiate tests using this logic for the supported VM versions. If necessary, extend the tested VM trait so
   that test logic can be defined in a generic way. See the `testonly` module docs for more detailed guidelines.
+- If you define a generic test, don't forget to add its instantiations for all supported VMs (`vm_latest`, `vm_fast` and
+  `shadow`). `shadow` tests allow checking VM divergences for free!
 - Do not use an RNG where it can be avoided (e.g., for test contract addresses).
 - Avoid using zero / default values in cases they can be treated specially by the tested code.

core/lib/multivm/src/versions/mod.rs

@@ -1,3 +1,5 @@
+#[cfg(test)]
+mod shadow;
 mod shared;
 #[cfg(test)]
 mod testonly;

core/lib/multivm/src/versions/testonly/shadow.rs → core/lib/multivm/src/versions/shadow/mod.rs

@@ -28,6 +28,8 @@ use crate::{
     vm_latest::HistoryEnabled,
 };
 
+mod tests;
+
 type ReferenceVm<S = InMemoryStorage> = vm_latest::Vm<StorageView<S>, HistoryEnabled>;
 type ShadowedFastVm<S = InMemoryStorage> = crate::vm_instance::ShadowedFastVm<S>;