[test] TreeDictionary.Keys: Remove stray print that’s flooding test output

[lorentey]

The test_isEqual_exhaustive test includes a stray print statement that is flooding test output with a myriad lines of useless data.

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.