Checkout testdata submodule to correct location #6

Job	Run time
build (3.8, ubuntu-latest, base)	16s
build (3.8, ubuntu-latest, optional)	17s
build (3.8, windows-latest, base)	55s
build (3.8, windows-latest, optional)	53s
build (3.9, ubuntu-latest, base)	15s
build (3.9, ubuntu-latest, optional)	16s
build (3.9, windows-latest, base)	1m 14s
build (3.9, windows-latest, optional)	41s
build (3.10, ubuntu-latest, base)	20s
build (3.10, ubuntu-latest, optional)	18s
build (3.10, windows-latest, base)	51s
build (3.10, windows-latest, optional)	46s
build (3.11, ubuntu-latest, base)	19s
build (3.11, ubuntu-latest, optional)	20s
build (3.11, windows-latest, base)	1m 11s
build (3.11, windows-latest, optional)	1m 12s
build (3.12, ubuntu-latest, base)	23s
build (3.12, ubuntu-latest, optional)	22s
build (3.12, windows-latest, base)	1m 32s
build (3.12, windows-latest, optional)	59s
build (pypy-3.10, ubuntu-latest, base)	36s
build (3.8, ubuntu-latest, oldest)	16s
	14m 12s

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