TST (string dtype): update tests/reductions tests #19900

Job	Run time
Linux-32-bit	19m 5s
Linux-Musl	16m 57s
Pyodide build	24m 37s
macos-13 actions-310.yaml	31m 41s
macos-13 actions-311.yaml	31m 9s
macos-13 actions-312.yaml	47m 11s
macos-14 actions-310.yaml	17m 2s
macos-14 actions-311.yaml	18m 18s
macos-14 actions-312.yaml	20m 0s
windows-latest actions-311.yaml	36m 39s
windows-latest actions-312.yaml	46m 46s
windows-latest actions-310.yaml	34m 4s
python-dev (macOS-13)	27m 31s
python-dev (macOS-14)	11m 1s
python-dev (ubuntu-22.04)	20m 55s
python-dev (windows-latest)	23m 39s
python-freethreading	23m 0s
ubuntu-latest actions-310.yaml	44m 3s
Minimum Versions	24m 19s
ubuntu-latest actions-311.yaml	39m 47s
Future infer strings (without pyarrow)	40m 11s
Locale: zh_CN	24m 49s
Locale: it_IT	24m 37s
Downstream Compat	6m 33s
Numpy Dev	17m 16s
Pyarrow Nightly	19m 54s
ubuntu-latest actions-312.yaml	46m 31s
Future infer strings	49m 43s
Pypy	1m 21s
	13h 8m 39s

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