feat: Swift 6 support and switch to swift-testing #216

Job	Run time
Mac Test (macOS, SWXMLHash OSX, build-for-testing test-without-building)	11s
Linux Test (swift:5.8-focal, true)	23s
Mac Test (iOS, SWXMLHash iOS, build-for-testing test-without-building, -sdk iphonesimulator -dest...	10s
Linux Test (swift:5.8-jammy, true)	28s
Mac Test (tvOS, SWXMLHash tvOS, -sdk appletvsimulator -destination "name=Apple TV")	11s
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)	31s
Linux Test (swift:5.9-jammy, true)	33s
Linux Test (swift:5.9-rhel-ubi9, true)	31s
Windows Test (swiftlang/swift:nightly-6.0-windowsservercore-1809)	1s
Linux Test (swift:5.10-focal, true)	31s
Linux Test (swift:5.10-jammy, true)	31s
Linux Test (swift:5.10-rhel-ubi9, true)	30s
Linux Test (swiftlang/swift:nightly-6.0-focal, true)	2m 43s
Linux Test (swiftlang/swift:nightly-6.0-jammy, true)	2m 37s
	10m 30s

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