test: fix flaky test-watch-mode-kill-signal-* #298

Job	Run time
Build slim tarball	1m 40s
aarch64-linux: with shared libraries	24m 28s
x86_64-linux: with shared libraries	28m 20s
aarch64-darwin: with shared libraries	30m 57s
x86_64-darwin: with shared libraries	41m 8s
	2h 6m 33s

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