fix the complie error with test_QUICStreamManager

[scw00]

root@ubuntu-linux:~/trafficserver/iocore/net/quic/test# make test_QUICStreamManager
  CXX      test_QUICStreamManager-test_QUICStreamManager.o
In file included from test_QUICStreamManager.cc:30:0:
/root/trafficserver/iocore/net/quic/Mock.h:116:3: error: ‘Ptr<ProxyMutex> MockQUICConnection::get_transmitter_mutex()’ marked override, but does not override
   get_transmitter_mutex() override
   ^
/root/trafficserver/iocore/net/quic/Mock.h:227:3: error: ‘Ptr<ProxyMutex> MockQUICPacketTransmitter::get_transmitter_mutex()’ marked override, but does not override
   get_transmitter_mutex() override
   ^
/root/trafficserver/iocore/net/quic/Mock.h: In constructor ‘MockQUICLossDetector::MockQUICLossDetector()’:
/root/trafficserver/iocore/net/quic/Mock.h:258:75: error: invalid new-expression of abstract class type ‘MockQUICPacketTransmitter’
   MockQUICLossDetector() : QUICLossDetector(new MockQUICPacketTransmitter()) {}
                                                                           ^
/root/trafficserver/iocore/net/quic/Mock.h:210:7: note:   because the following virtual functions are pure within ‘MockQUICPacketTransmitter’:
 class MockQUICPacketTransmitter : public QUICPacketTransmitter
       ^
In file included from /root/trafficserver/iocore/net/quic/QUICConnection.h:26:0,
                 from /root/trafficserver/iocore/net/P_QUICNetVConnection.h:45,
                 from /root/trafficserver/iocore/net/P_Net.h:113,
                 from /root/trafficserver/iocore/net/quic/QUICLossDetector.h:34,
                 from /root/trafficserver/iocore/net/quic/Mock.h:27,
                 from test_QUICStreamManager.cc:30:
/root/trafficserver/iocore/net/quic/QUICPacketTransmitter.h:49:27: note: 	virtual Ptr<ProxyMutex> QUICPacketTransmitter::get_packet_transmitter_mutex()
   virtual Ptr<ProxyMutex> get_packet_transmitter_mutex() = 0;
                           ^
In file included from test_QUICStreamManager.cc:30:0:
/root/trafficserver/iocore/net/quic/Mock.h: In constructor ‘MockQUICApplication::MockQUICApplication()’:
/root/trafficserver/iocore/net/quic/Mock.h:361:47: error: invalid new-expression of abstract class type ‘MockQUICConnection’
   MockQUICApplication() : QUICApplication(new MockQUICConnection) { SET_HANDLER(&MockQUICApplication::main_event_handler); }
                                               ^
/root/trafficserver/iocore/net/quic/Mock.h:96:7: note:   because the following virtual functions are pure within ‘MockQUICConnection’:
 class MockQUICConnection : public QUICConnection
       ^
In file included from /root/trafficserver/iocore/net/quic/QUICConnection.h:26:0,
                 from /root/trafficserver/iocore/net/P_QUICNetVConnection.h:45,
                 from /root/trafficserver/iocore/net/P_Net.h:113,
                 from /root/trafficserver/iocore/net/quic/QUICLossDetector.h:34,
                 from /root/trafficserver/iocore/net/quic/Mock.h:27,
                 from test_QUICStreamManager.cc:30:
/root/trafficserver/iocore/net/quic/QUICPacketTransmitter.h:49:27: note: 	virtual Ptr<ProxyMutex> QUICPacketTransmitter::get_packet_transmitter_mutex()
   virtual Ptr<ProxyMutex> get_packet_transmitter_mutex() = 0;
                           ^
At global scope:
cc1plus: warning: unrecognized command line option "-Wno-format-truncation"
make: *** [test_QUICStreamManager-test_QUICStreamManager.o] Error 1