testrunner: made `-x` work with individual tests #19147

Job	Run time
build_cmake (alpine:3.23, false, false)	4m 4s
build_make (alpine:3.23)	6m 7s
build_cmake (ubuntu:24.04, true, true)	5m 39s
build_make (ubuntu:24.04)	3m 48s
build_cmake (ubuntu:25.10, true, true)	3m 41s
build_make (ubuntu:25.10)	5m 25s
	28m 44s

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