Fix flaky test

Author: gjcairo

Tests/NIOHTTPServerTests/NIOHTTPServer+ServiceLifecycleTests.swift

@@ -184,6 +184,13 @@ struct NIOHTTPServiceLifecycleTests {
                     // Wait for the server to shut down.
                     try await group.waitForAll()
 
+                    // Wait for the client channel to be fully closed. The server has closed
+                    // its side of the connection, but the client's event loop may not have
+                    // processed the TCP FIN/RST yet. closeFuture completes only once the
+                    // channel is fully inactive, which is a stronger guarantee than just
+                    // draining inbound (which may return while the channel is half-closed).
+                    try await client.channel.closeFuture.get()
+
                     // We shouldn't be able to complete our request; the server should have shut down.
                     await #expect(throws: ChannelError.ioOnClosedChannel) {
                         try await outbound.write(Self.reqBody)