RISC-V: Convert tests using legacy in-memory backend

Closes RV-210

What

Migrates tests that previously ran against the legacy in-memory backend to the new Owned backend manager. Also retires the in-memory backend as a testing backend.

Why

We want to make sure all our tests work properly against the Owned backend manager.

How

I've split the work into two commits to make repetitive changes (2nd commit) easier to review.

Manually Testing

make -C src/riscv all

Benchmarking

This doesn't affect performance. ¡

Tasks for the Author

• Link all Linear issues related to this MR using magic words (e.g. part of, relates to, closes).
• Eliminate dead code and other spurious artefacts introduced in your changes.
• Document new public functions, methods and types.
• Make sure the documentation for updated functions, methods, and types is correct.
• Add tests for bugs that have been fixed.
• Put in reasonable effort to ensure that CI will pass.
• Benchmark performance and populate the table above if needed.
• Write commit messages to reflect the changes they're about.
• Self-review your changes to ensure they are high-quality.
• Complete all of the above before assigning this MR to reviewers.