docs: clarify test file structure convention in CLAUDE.md (#2006) #1

Job	Run time
checks / pre-commit	49s
checks / readme-snippets	14s
checks / test (3.10, locked, ubuntu-latest)	54s
checks / test (3.11, lowest-direct, ubuntu-latest)	53s
checks / test (3.11, lowest-direct, windows-latest)	2m 38s
checks / test (3.10, lowest-direct, windows-latest)	2m 33s
checks / test (3.12, lowest-direct, windows-latest)	3m 44s
checks / test (3.11, locked, windows-latest)	2m 41s
checks / test (3.13, lowest-direct, windows-latest)	3m 8s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 11s
checks / test (3.14, locked, ubuntu-latest)	2m 17s
checks / test (3.14, locked, windows-latest)	3m 51s
checks / test (3.14, lowest-direct, windows-latest)	3m 8s
checks / test (3.13, lowest-direct, ubuntu-latest)	50s
checks / test (3.13, locked, windows-latest)	2m 24s
checks / test (3.13, locked, ubuntu-latest)	45s
checks / test (3.12, locked, ubuntu-latest)	53s
checks / test (3.10, lowest-direct, ubuntu-latest)	52s
checks / test (3.12, locked, windows-latest)	3m 33s
checks / test (3.11, locked, ubuntu-latest)	48s
checks / test (3.10, locked, windows-latest)	2m 29s
checks / test (3.12, lowest-direct, ubuntu-latest)	57s
all-green	5s
	41m 37s

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