test: fix flaky test-http2-close-while-writing #3354

Job	Run time
Build slim tarball	2m 22s
x86_64-darwin: with shared libraries	1h 17m 4s
aarch64-darwin: with shared libraries	53m 55s
x86_64-linux: with shared libraries	58m 26s
aarch64-linux: with shared libraries	41m 26s
	3h 53m 13s

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