Add test case for unreached fixpoint due to bug in congruences showin… #12928

Job	Run time
regression (ubuntu-22.04, ocaml-variants.4.14.2+options,ocaml-option-flambda)	8m 47s
regression (macos-13, ocaml-variants.4.14.2+options,ocaml-option-flambda)	29m 19s
extraction	0s
gobview (ubuntu-22.04, ocaml-variants.4.14.2+options,ocaml-option-flambda, 14)	7m 31s
	45m 37s

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