test: fix flaky test-watch-mode-kill-signal-* #312

Job	Run time
Build slim tarball	1m 31s
x86_64-linux: with shared libraries	4m 34s
aarch64-linux: with shared libraries	4m 30s
x86_64-darwin: with shared libraries	4m 42s
aarch64-darwin: with shared libraries	4m 25s
	19m 42s

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