Fix flaky test in InputStreamStreamMessageTest

[kojilin]

Close #5695

[github-actions]

🔍 Build Scan® (commit: 7ab287b)

Job name	Status	Build Scan®
build-windows-latest-jdk-21	✅	https://ge.armeria.dev/s/a62jh63nifs2m
build-self-hosted-unsafe-jdk-8	✅	https://ge.armeria.dev/s/ywstotwh46266
build-self-hosted-unsafe-jdk-21-snapshot-blockhound	✅	https://ge.armeria.dev/s/tie6634o2fya4
build-self-hosted-unsafe-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/4mueycqb3xnve
build-self-hosted-unsafe-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/oz5a6ahgk3shg
build-self-hosted-unsafe-jdk-17-leak	✅	https://ge.armeria.dev/s/murgzj2oytvvq
build-self-hosted-unsafe-jdk-11	✅	https://ge.armeria.dev/s/te5skufautba2
build-macos-12-jdk-21	✅	https://ge.armeria.dev/s/3ph5asrpd2ye6

[minwoox]

Great, thanks!