Adopt Swift Testing

[SimplyDanny]

Swift Testing requires all builds to run on Swift 6.1.

[SwiftLintBot]

	2 Warnings
⚠️	Big PR
⚠️	If this is a user-facing change, please include a CHANGELOG entry to credit yourself! You can find it at CHANGELOG.md.

	2 Messages
📖	Building this branch resulted in the same binary size as when built on main.
📖	Skipping OSS check because SwiftLint hasn't changed compared to main.

Here's an example of your CHANGELOG entry:

* Adopt Swift Testing.  
  [SimplyDanny](https://github.com/SimplyDanny)
  [#issue_number](https://github.com/realm/SwiftLint/issues/issue_number)

note: There are two invisible spaces after the entry's text.

Generated by 🚫 Danger