test: apply a delay to watch-mode-kill-sginal tests #670

Job	Run time
Build slim tarball	1m 37s
aarch64-linux: with shared libraries	24m 9s
aarch64-darwin: with shared libraries	36m 10s
x86_64-linux: with shared libraries	28m 41s
x86_64-darwin: with shared libraries	59m 4s
	2h 29m 41s

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