Fix flaky InitiateConnectionShutdownTest (#5944)

Motivation: 

- #5743
- #5333
- #3982

I'm not sure this change could resolve the flakiness but it would be
worthwhile to explicitly wait for a connection to close before checking
invocations.

Modifications:

- Wait for `clientChannel` to be closed.

Result:

- Closes #5743
- Closes #5333
- Closes #3982

I will reopen the issues if the flaky is reproduced.

core/src/test/java/com/linecorp/armeria/server/InitiateConnectionShutdownTest.java

@@ -142,6 +142,7 @@ private void makeHttp2Request(String path) throws Exception {
         });
         await().untilTrue(finished);
         await().untilTrue(connectionClosed);
+        clientChannel.closeFuture().syncUninterruptibly();
     }
 
     @BeforeEach