Improve test case folder structure; tweak code style #237

Job	Run time
Test Python 3.8 (ubuntu-latest)	1m 26s
Test Python 3.9 (ubuntu-latest)	1m 19s
Test Python 3.10 (ubuntu-latest)	1m 25s
Test Python 3.11 (ubuntu-latest)	1m 37s
Test Python 3.8 (windows-latest)	6m 12s
Test Python 3.9 (windows-latest)	3m 46s
Test Python 3.10 (windows-latest)	3m 30s
Test Python 3.11 (windows-latest)	5m 7s
Test Python 3.8 (macOS-latest)	2m 3s
Test Python 3.9 (macOS-latest)	6m 1s
Test Python 3.10 (macOS-latest)	1m 58s
Test Python 3.11 (macOS-latest)	2m 3s
Test pre-commit console encoding	1m 29s
	37m 56s

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