[test] Use @retroactive in test conformances

[lorentey]

The swift-collections test suite includes some test protocols to validate that dictionary- and set-like types provide the API surfaces that’s expected of their kind. These are being conformed to in the test targets, triggering new warnings on retroactive conformances.

Use @retroactive to silence such warnings.

Checklist

• [X] I've read the Contribution Guidelines
• [X] My contributions are licensed under the Swift license.
• [X] I've followed the coding style of the rest of the project.
• [ ] I've added tests covering all new code paths my change adds to the project (if appropriate).
• [ ] I've added benchmarks covering new functionality (if appropriate).
• [X] I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• [ ] I've updated the documentation if necessary.

[lorentey]

(I expect this will fail on Swift 5.7, possibly 5.8.)

[lorentey]

@swift-ci test