test: fix flaky keepalive timeout test on slow platforms #787

Job	Run time
Build slim tarball	1m 34s
x86_64-linux: with shared libraries	27m 52s
aarch64-darwin: with shared libraries	28m 53s
x86_64-darwin: with shared libraries	35m 28s
aarch64-linux: with shared libraries	23m 57s
	1h 57m 44s

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