Fix flaky tests

[ikhoon]

Motivation:

Three tests often fail in CI builds.

• Test failure: com.linecorp.armeria.common.stream.StreamMessageCollectingTest.filteredStreamMessage_cancel() #5942
• Test failure: com.linecorp.armeria.server.Http1ServerDelayedCloseConnectionTest.shouldWaitForDisconnectByClientSideFirst() #5724
• Test failure: com.linecorp.armeria.client.DefaultDnsCacheTest.eviction() #4960

The cause of the failures seems to be a timing issue.

Modifications:

• Wrap assertion with await().untilAsserted()
• Perform get operations to evict the old caches.

Result:

• Fixes Test failure: com.linecorp.armeria.common.stream.StreamMessageCollectingTest.filteredStreamMessage_cancel() #5942
• Fixes Test failure: com.linecorp.armeria.server.Http1ServerDelayedCloseConnectionTest.shouldWaitForDisconnectByClientSideFirst() #5724
• Fixes Test failure: com.linecorp.armeria.client.DefaultDnsCacheTest.eviction() #4960

[github-actions]

🔍 Build Scan® (commit: fe294b2)

Job name	Status	Build Scan®
build-ubicloud-standard-8-jdk-8	✅	https://ge.armeria.dev/s/simsijyi3sjwc
build-ubicloud-standard-8-jdk-21-snapshot-blockhound	✅	https://ge.armeria.dev/s/vap3knngf2lyq
build-ubicloud-standard-8-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/q5fyd77yoc4vm
build-ubicloud-standard-8-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/nliu4xxcygexm
build-ubicloud-standard-8-jdk-17-leak	✅	https://ge.armeria.dev/s/capldjvthyfpm
build-ubicloud-standard-8-jdk-11	❌ (failure)	https://ge.armeria.dev/s/zitbx3mhbkzzq
build-macos-12-jdk-21	✅	https://ge.armeria.dev/s/xq77kc4pwokjy