test: fix flaky test-watch-mode-kill-signal-* #288

Job	Run time
Build slim tarball	1m 19s
x86_64-linux: with shared libraries	1h 48m 23s
aarch64-darwin: with shared libraries	1h 24m 35s
aarch64-linux: with shared libraries	1h 32m 31s
x86_64-darwin: with shared libraries	1h 59m 22s
	6h 46m 10s

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