Clean up orchard test submodule layout

[teor2345]

Motivation

Zebra is moving towards a standard test code layout, where:

• Arbitrary imps are in their own module
• Tests are submodules of a tests module

This speeds up non-test builds, particularly when only tests have changed.

But the orchard module in #1885 doesn't quite follow this structure yet.

Suggested Changes

Move to an arbitrary module in its own file:

• [ ] Arbitrary for Address
• [ ] Arbitrary for Halo2Proof
• [ ] Arbitrary for TransmissionKey
  • also make it depend on test or proptest-impl

Move to tests::{prop, vectors}, with each module in its own file:

• [ ] address::tests
• [ ] commitment::tests
• [ ] note::ciphertexts::tests
• [ ] note::nullifiers::tests

[teor2345]

We can do this cleanup if it is ever needed.