Rename integration test crates and extract shared code into a separate crate

Following https://github.com/0xMiden/compiler/pull/555#discussion_r2248146332 and https://github.com/0xMiden/compiler/pull/555#discussion_r2248158572

1. Rename `miden-integration-tests` to `midenc-integration-tests`;
2. Extract all the exports into a shared `midenc-testing-crate`;