Allow runtest to run individual tests #28834

Job	Run time
Coq 8.16.1	7m 17s
Coverage (5.1.x)	9m 49s
Build (macos-latest, 5.1.x)	13m 46s
Build (ubuntu-latest, 5.1.x)	9m 22s
Documentation	51s
Create local opam switch (macos-latest, 5)	6m 13s
Build (ocaml-variants.5.3.0+trunk, ubuntu-latest, true)	5m 16s
Create local opam switch (macos-latest, 4.14)	5m 47s
Build (4.14.x, ubuntu-latest, true)	1m 46s
Create local opam switch (ubuntu-latest, 5)	5m 2s
Format	1m 0s
Build (4.14.x, macos-latest, true)	2m 47s
Build monorepo benchmark docker image	3m 56s
Create local opam switch (ubuntu-latest, 4.14)	5m 12s
Build (5.1.x, macos-13, true)	5m 22s
Build (ocaml-base-compiler.4.14.2,system-msvc, windows-latest)	16m 18s
Build (ocaml-base-compiler.5.1.1,system-mingw, windows-latest)	10m 21s
Build microbenchmarks	2m 20s
Nix (macos-latest)	2m 2s
Build (4.08.x, ubuntu-latest, true)	1m 44s
Nix (ubuntu-latest)	52s
Build (4.04.x, ubuntu-latest, true, true)	5m 37s
Build (4.02.x, ubuntu-latest, true)	4m 5s
	2h 6m 45s

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