refactor: hoist up some test stanza helpers (#11084) #28896

Job	Run time
Nix (macos-latest)	47s
Build (macos-latest, 5.1.x)	1s
Nix (ubuntu-latest)	57s
Build (ubuntu-latest, 5.1.x)	3m 8s
Coverage (5.1.x)	3m 14s
Build (ocaml-variants.5.3.0+trunk, ubuntu-latest, true)	3m 12s
Format	1m 0s
Build (4.14.x, ubuntu-latest, true)	1m 40s
Build (4.14.x, macos-latest, true)	1s
Build (5.1.x, macos-13, true)	47s
Build (ocaml-base-compiler.4.14.2,system-msvc, windows-latest)	2m 57s
Build (ocaml-base-compiler.5.1.1,system-mingw, windows-latest)	2m 59s
Build microbenchmarks	2m 19s
Build (4.08.x, ubuntu-latest, true)	1m 35s
Build (4.04.x, ubuntu-latest, true, true)	3m 11s
Coq 8.16.1	3m 18s
Build (4.02.x, ubuntu-latest, true)	3m 13s
Documentation	49s
Create local opam switch (macos-latest, 5)	1s
Create local opam switch (macos-latest, 4.14)	1s
Build monorepo benchmark docker image	3m 15s
Create local opam switch (ubuntu-latest, 5)	3m 11s
Create local opam switch (ubuntu-latest, 4.14)	3m 11s
	44m 47s

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