refactor: move runtest to own module #28787

Job	Run time
Nix (macos-latest)	1m 54s
Build (macos-latest, 5.1.x)	12m 55s
Nix (ubuntu-latest)	52s
Build microbenchmarks	2m 15s
Build (ubuntu-latest, 5.1.x)	8m 53s
Coq 8.16.1	7m 18s
Build (ocaml-variants.5.3.0+trunk, ubuntu-latest, true)	4m 50s
Build (4.14.x, ubuntu-latest, true)	1m 35s
Coverage (5.1.x)	10m 0s
Build (4.14.x, macos-latest, true)	2m 22s
Create local opam switch (macos-latest, 5)	8m 32s
Format	1m 6s
Build (5.1.x, macos-13, true)	4m 11s
Create local opam switch (macos-latest, 4.14)	5m 29s
Build (ocaml-base-compiler.4.14.2,system-msvc, windows-latest)	16m 34s
Build monorepo benchmark docker image	3m 50s
Create local opam switch (ubuntu-latest, 5)	5m 8s
Build (ocaml-base-compiler.5.1.1,system-mingw, windows-latest)	10m 41s
Documentation	52s
Build (4.08.x, ubuntu-latest, true)	2m 19s
Create local opam switch (ubuntu-latest, 4.14)	5m 1s
Build (4.04.x, ubuntu-latest, true, true)	4m 47s
Build (4.02.x, ubuntu-latest, true)	4m 10s
	2h 5m 34s

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