pre-built MASP proofs for integration tests

[tzemanovic]

Describe your changes

To speed-up MASP integration tests that were timing out in CI, this PR adds the ability to save and load proofs to/from files stored in test_fixtures/masp_proofs.

# Run the tests with the saved proofs.
make test-integration

# Delete old proofs, run the tests and save the new proofs.
# Use this is there's a breaking change in either the proof creation or 
# in the MASP integration tests that invalidates the previously saved proofs.
make test-integration-save-proofs

# Run without pre-built proofs
make test-integration-slow

The prebuilt proofs are added to make test-coverage to allow running integration tests in the CI.

The MASP proof builder in fn gen_shielded_transfer is optionally given a seed to produce deterministic output. When not specified, the original behavior is preserved (i.e. using OsRng, but now wrapped in StdRng which is cryptographically secure - see https://rust-random.github.io/rand/rand/trait.SeedableRng.html#method.from_rng for more details).

To enable deterministic builders, the Conversions type and notes positions in ShieldedContext are changed to BTreeMap and BTreeSet resp to provide deterministic output from fn collect_unspent_notes.

Indicate on which release or other PRs this topic is based on

0.20.0

Checklist before merging to draft

• I have added a changelog
• Git history is in acceptable state

[Fraccaman]

this is awesome! Minor thing: can we remove the masp test from the e2e.json file?

[tzemanovic]

this is awesome! Minor thing: can we remove the masp test from the e2e.json file?

cheers! It is already removed on main but this branch is based on 0.20 so it still appears here