coreutils: split function parse + add tests #1951

Job	Run time
Style/cargo-deny	19s
Code Coverage (ubuntu-latest, unix, nightly)	1m 17s
Dependencies (ubuntu-latest, feat_os_unix)	12s
Code Coverage (macos-latest, macos, nightly)	42s
Documentation/warnings (ubuntu-latest, feat_os_unix)	1m 14s
Code Coverage (windows-latest, windows, nightly-x86_64-pc-windows-gnu)	1m 8s
MinRustV (ubuntu-latest, feat_os_unix)	1m 15s
Style/deps (ubuntu-latest, feat_Tier1,feat_require_unix,feat_require_unix_utmpx)	56s
Style/deps (macos-latest, feat_Tier1,feat_require_unix,feat_require_unix_utmpx)	1m 9s
Style/deps (windows-latest, feat_os_windows)	1m 6s
Build/nightly	1s
Build/Makefile	1s
Build	1s
Build/stable	1s
Binary sizes	1s
Tests/BusyBox test suite	1s
Tests/Toybox test suite	1s
	9m 25s

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