Test FileManager against simulated server errors

Specifically much better testing for the handling of:
- Ack responses not being sent by server
- Getting Nak’s back from commands
- CRC failures

Also changed file validation to take into account Mock Server no longer
sending file length in first byte of file.

src/qgcunittest/QGCUASFileManagerTest.h

@@ -87,6 +87,10 @@ private:
     static const size_t _cSignals = maxSignalIndex;
     const char*         _rgSignals[_cSignals];
     
+    /// @brief This is the amount of time to wait to allow the FileManager enough time to timeout waiting for an Ack.
+    /// As such it must be larger than the Ack Timeout used by the FileManager.
+    static const int _ackTimerTimeoutMsecs = QGCUASFileManager::ackTimerTimeoutMsecs * 2;
+    
     QStringList _fileListReceived;
 };