Adopt swift-testing for AttributedString tests #132

Job	Run time
Test / Windows (nightly-main - windows-2019)	20m 42s
Test / Linux (nightly-main - jammy)	10m 56s
Test / macOS (Xcode ${{ matrix.xcode_version }} - ${{ matrix.os_version }} - ${{ matrix.arch }})	0s
Soundness / API breakage check	0s
Soundness / Broken symlinks check	4s
Soundness / Documentation check	0s
Soundness / Format check	0s
Soundness / License headers check	0s
Soundness / Python lint check	6s
Soundness / Shell check	36s
Soundness / Unacceptable language check	0s
Soundness / YAML lint check	4s
	32m 28s

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