feat: Swift 6 support and switch to swift-testing #218

Job	Run time
Mac Test (macOS, SWXMLHash OSX, build-for-testing test-without-building)	11s
Linux Test (swift:5.8-focal, true)	28s
Mac Test (iOS, SWXMLHash iOS, build-for-testing test-without-building, -sdk iphonesimulator -dest...	11s
Linux Test (swift:5.8-jammy, true)	31s
Mac Test (tvOS, SWXMLHash tvOS, -sdk appletvsimulator -destination "name=Apple TV")	10s
Linux Test (swift:5.8-rhel-ubi9, true)	29s
Mac Test (watchOS, SWXMLHash watchOS, build-for-testing test-without-building, -sdk watchsimulator)	10s
Linux Test (swift:5.9-focal, true)	30s
Linux Test (swift:5.9-jammy, true)	32s
Linux Test (swift:5.9-rhel-ubi9, true)	32s
Windows Test (swift-5.10-release, 5.10-RELEASE, true)	1m 39s
Linux Test (swift:5.10-focal, true)	30s
Windows Test (swift-6.0-branch, 6.0-DEVELOPMENT-SNAPSHOT-2024-09-11-a, true)	5m 22s
Linux Test (swift:5.10-jammy, true)	35s
Linux Test (swift:5.10-rhel-ubi9, true)	31s
Linux Test (swiftlang/swift:nightly-6.0-focal, true)	2m 45s
Linux Test (swiftlang/swift:nightly-6.0-jammy, true)	2m 39s
	17m 45s

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