test: fix flaky test-http2-close-while-writing #3260

Job	Run time
Build slim tarball	2m 29s
x86_64-darwin: with shared libraries	59m 35s
aarch64-darwin: with shared libraries	33m 39s
x86_64-linux: with shared libraries	37m 1s
aarch64-linux: with shared libraries	30m 48s
	2h 43m 32s

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