AbstractTransportTest.openStreamPreventsTermination is flaky on Windows

Seen in https://grpc-testing.appspot.com/job/gRPC-Java-PR-Windows/2337/console

```
io.grpc.netty.NettyTransportTest > openStreamPreventsTermination FAILED
java.io.IOException: Failed to bind

Caused by:
java.net.BindException: Address already in use: bind

java.lang.AssertionError

187 tests completed, 1 failed, 3 skipped
:grpc-netty:test FAILED
```