test: better testing around installs

[beeauvin]

Doesn't test caching but at least gets us closer to making sure the right version is installed :P

[Copilot]

Pull request overview

This PR enhances the testing framework for the Swiftly Swift action by adding version verification capabilities. The action now exposes the resolved Swift version as an output, allowing consumers and tests to verify that the correct version was installed.

• Adds a swift-version output to the action that exposes the resolved version
• Updates tests to verify the installed Swift version matches the resolved version output
• Expands test matrix to include major version "6" alongside "latest" and "6.1"

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
action.yml	Adds swift-version output that references the existing resolve-version step
.github/workflows/tests.yml	Updates test matrix to include version "6" and adds verification logic using the new output to confirm correct Swift version installation

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