Add extensive coverage for mrf-tool

[aumetra]

This is in preparation for actually writing a draft FEP and providing reliable and reproducible test vectors.
The CLI is supposed to be the flagship tooling for WASM MRFs and I want it to be in a highly tested and well working state.

TODO

• File system mocking
• Separate implementation into a lib.rs file so tests can hook into it
• Add tests

[aumetra]

Now the heart of the tool has like ~98% test coverage. Perfect.