testrunner: made `-x` work with individual tests (#8362) #19218

Job	Run time
build_cmake (alpine:3.23, false, false)	4m 24s
build_make (alpine:3.23)	5m 28s
build_make (ubuntu:24.04)	4m 24s
build_cmake (ubuntu:25.10, true, true)	5m 32s
build_cmake (ubuntu:24.04, true, true)	5m 47s
build_make (ubuntu:25.10)	5m 23s
	30m 58s

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