test: expand VM unit test coverage

[Jim8y]

Summary

• add execution snapshot helpers and execute-vs-debugger equivalence tests
• add engine/context invariants, limits coverage, and nightly deterministic fuzz tests
• fix bulk reference counting overload and align optimization test expectations

Test Plan

• dotnet test tests/Neo.VM.Tests/Neo.VM.Tests.csproj
• dotnet test tests/Neo.VM.Tests/Neo.VM.Tests.csproj --filter TestCategory=Nightly