Comment (by Roland Haas):

I accidentally pushed the version without fallback when no parallel tests are requested. Fixed (and an inconsistency when reporting exit codes addressed) in git hash c9922081 "Cactus: report correct error code of child shell in parallel tests" of cactus

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2463/add-option-to-run-tests-in-parallel