feat: bump to Swift 6, add swift-testing #221

Job	Run time
Mac Test (macOS, SWXMLHash OSX, build-for-testing test-without-building)	11s
Linux Test (swift:5.8-focal, true)	27s
Mac Test (iOS, SWXMLHash iOS, build-for-testing test-without-building, -sdk iphonesimulator -dest...	11s
Windows Test (swift-5.10-release, 5.10-RELEASE, true)	1m 28s
Linux Test (swift:5.8-jammy, true)	28s
Mac Test (tvOS, SWXMLHash tvOS, -sdk appletvsimulator -destination "name=Apple TV")	11s
Windows Test (swift-6.0-release, 6.0-RELEASE, true)	5m 19s
Linux Test (swift:5.8-rhel-ubi9, true)	27s
Mac Test (watchOS, SWXMLHash watchOS, build-for-testing test-without-building, -sdk watchsimulator)	14s
Linux Test (swift:5.9-focal, true)	28s
Linux Test (swift:5.9-jammy, true)	28s
Linux Test (swift:5.9-rhel-ubi9, true)	32s
Linux Test (swift:5.10-focal, true)	28s
Linux Test (swift:5.10-jammy, true)	31s
Linux Test (swift:5.10-rhel-ubi9, true)	31s
Linux Test (swift:6.0-focal, true)	2m 12s
Linux Test (swift/6.0-jammy, true)	9s
Linux Test (swift:6.0-rhel-ubi9, true)	2m 12s
	16m 27s

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