HB-relationship involving thread creations while mutexes are held #15138

Job	Run time
regression (macos-latest, ocaml-variants.4.14.2+options,ocaml-option-flambda)	13m 38s
regression (ubuntu-latest, ocaml-variants.4.14.2+options,ocaml-option-flambda)	7m 46s
extraction	0s
	21m 24s

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