The PipeSocket was convenient and avoided real I/O, but the
shutdown/close while connecting/handshaking tests were triggering a
Socket bug in Java (https://bugs.openjdk.org/browse/JDK-8278326). Using
a real socket doesn't trigger the bug because the test stops sharing
state with the code under test.

Fixes grpc#10228

```
Details
==================
WARNING: ThreadSanitizer: data race (pid=4528)
  Write of size 1 at 0x0000cfb9d5f4 by thread T36 (mutexes: write M0):
    #0 java.net.Socket.setCreated()V Socket.java:687
    #1 java.net.AbstractPlainSocketImpl.create(Z)V AbstractPlainSocketImpl.java:149
    #2 java.net.Socket.createImpl(Z)V Socket.java:477
    #3 java.net.Socket.getImpl()Ljava/net/SocketImpl; Socket.java:540
    #4 java.net.Socket.setTcpNoDelay(Z)V Socket.java:998
    #5 io.grpc.okhttp.OkHttpServerTransport.startIo(Lio/grpc/internal/SerializingExecutor;)V OkHttpServerTransport.java:164
    #6 io.grpc.okhttp.OkHttpServerTransport.lambda$start$0(Lio/grpc/internal/SerializingExecutor;)V OkHttpServerTransport.java:159
    #7 io.grpc.okhttp.OkHttpServerTransport$$Lambda$56.run()V ??
    #8 io.grpc.internal.SerializingExecutor.run()V SerializingExecutor.java:133
    grpc#9 java.util.concurrent.ThreadPoolExecutor.runWorker(Ljava/util/concurrent/ThreadPoolExecutor$Worker;)V ThreadPoolExecutor.java:1130
    grpc#10 java.util.concurrent.ThreadPoolExecutor$Worker.run()V ThreadPoolExecutor.java:630
    grpc#11 java.lang.Thread.run()V Thread.java:830
    grpc#12 (Generated Stub) <null>

  Previous read of size 1 at 0x0000cfb9d5f4 by thread T35 (mutexes: write M1, write M2):
    #0 java.net.Socket.close()V Socket.java:1512
    #1 io.grpc.okhttp.OkHttpServerTransportTest$PipeSocket.close()V OkHttpServerTransportTest.java:1384
    #2 io.grpc.okhttp.OkHttpServerTransportTest.clientCloseDuringHandshake()V OkHttpServerTransportTest.java:290
```