test(vm): Deduplicate multivm tests (#3109)

## What ❔

Deduplicates unit tests in the `multivm` crate by retaining a single
generic copy parameterized by a VM (with test-specific functionality
encapsulated in a new `TestedVm` trait).

## Why ❔

2 copies of almost every test is difficult to maintain.

## 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

@@ -4,3 +4,15 @@ This crate represents a wrapper over several versions of VM that have been used
 glue code that allows switching the VM version based on the externally provided marker while preserving the public
 interface. This crate exists to enable the external node to process breaking upgrades and re-execute all the
 transactions from the genesis block.
+
+## Developer guidelines
+
+### Adding tests
+
+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.
+- 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

@@ -1,8 +1,6 @@
 mod shared;
 #[cfg(test)]
 mod testonly;
-#[cfg(test)]
-mod tests;
 pub mod vm_1_3_2;
 pub mod vm_1_4_1;
 pub mod vm_1_4_2;