[Utils] run-full-tests.sh: Add support for testing on Swift 5.3 without manual editing

[lorentey]

The version check is a bit of a hack, but this enables test discovery on 5.3 so the script can be used to test the package on that release without manual tweaking.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• 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).
• 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]

@swift-ci test

[lorentey]

@swift-ci test