Make testConnectionDropWithOpenStreams more reliable (#1922)

Motivation:

'testConnectionDropWithOpenStreams' occasionally fails by going into
the idle state rather than transient failure when the connection is
dropped. This happens because the connection doesn't believe there to be
any streams open. There is a race here between the stream being opened
and NIO firing the stream opened event. If the connection is dropped
between those two events then the test will fail.

Modifications:

- Wait for the server to respond with metadata before dropping the
  connection

Result:

More stability

Tests/GRPCHTTP2CoreTests/Client/Connection/LoadBalancers/SubchannelTests.swift

@@ -396,11 +396,7 @@ final class SubchannelTests: XCTestCase {
       }
 
       group.addTask {
-        try await server.run { inbound, outbound in
-          // Sleep, the RPC will be cancelled at the end of the test because the connection
-          // will be purposefully dropped.
-          try await Task.sleep(for: .seconds(300))
-        }
+        try await server.run(.echo)
       }
 
       var events = [Subchannel.Event]()
@@ -421,11 +417,17 @@ final class SubchannelTests: XCTestCase {
             let stream = try await subchannel.makeStream(descriptor: .echoGet, options: .defaults)
             try await stream.execute { inbound, outbound in
               try await outbound.write(.metadata([:]))
+
+              // Wait for the metadata to be echo'd back.
+              var iterator = inbound.makeAsyncIterator()
+              let _ = try await iterator.next()
+
               // Stream is definitely open. Bork the connection.
               server.clients.first?.close(mode: .all, promise: nil)
-              for try await _ in inbound {
-                ()
-              }
+
+              // Wait for the next message which won't arrive, client won't send a message. The
+              // stream should fail
+              let _ = try await iterator.next()
             }
           } else if readyCount == 2 {
             subchannel.close()