executor: set PR_SET_PDEATHSIG for TestOS #11866

Job	Run time
dashboard	2m 17s
aux	4m 3s
build	7m 2s
arch (presubmit_arch_linux)	7m 27s
old	4m 13s
arch (presubmit_arch_freebsd)	5m 24s
race_dashboard	2m 45s
race	6m 51s
arch (presubmit_arch_netbsd)	3m 36s
arch (presubmit_arch_openbsd)	3m 39s
arch (presubmit_arch_darwin)	3m 28s
arch (presubmit_arch_windows)	2m 42s
arch (presubmit_arch_executor)	2m 50s
	56m 17s

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