test(vm): Deduplicate multivm tests

[slowli]

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

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

[slowli]

The change diff is smaller than it looks; it mostly consists of moving stuff around. I've needed to massage a couple of tests so that they work in the same way for both legacy and fast VMs, but I've tried to not reduce the test scope whenever possible.