SOCKET_ERROR_ABORT (2) when closing stream at the end of test.

[mazimkhan]

See line 04:09:15 [1456978147.91][CONN][RXD] MBED: Test finished!MBED: Socket Error: Connection aborted (20) in the log http://e108747.cambridge.arm.com:8080/job/gtt/232/console

Issue is in test tcp_echo_client. Here the stream is closed at the end of the test. This causes onError() handler call with socket error SOCKET_ERROR_ABORT

[ciarmcom]

ARM Internal Ref: IOTSFW-2225