dirname: add fuzzer to test GNU compatibility #17361

Job	Run time
Style/format (ubuntu-latest, feat_os_unix)	48s
Style/spelling (ubuntu-latest, feat_os_unix)	17s
Pre-commit hooks	1m 9s
Style/Python	7s
Style/toml	11s
Style/lint (ubuntu-latest, all, true)	2m 9s
Style/lint (macos-latest, feat_os_macos)	1m 52s
Style/lint (windows-latest, feat_os_windows)	4m 9s
	10m 42s

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