LogStream: Added a testing mode to the simgear logstreams.

The new function logstream::setTestingMode() has been created to allow for a
testing mode to be set.  This stores a boolean in the private logstream thread
which modifies the behaviour of the would_log() function, allowing for
everything to be logged.  The removeCallbacks() function has also been added,
allowing for the default fgfs output to STDOUT and STDERR to be silenced when
calling setTestingMode().

simgear/debug/logstream.hxx

@@ -158,12 +158,22 @@ public:
      
     void removeCallback(simgear::LogCallback* cb);
 
+    void removeCallbacks();
+
     /**
      * optionally record all entries and submit them to new log callbacks that
      * are added. This allows simplified logging configuration, but still including
      * early startup information in all logs.
      */
     void setStartupLoggingEnabled(bool enabled);
+
+    /**
+     * Set up the logstream for running in test mode.  For example the callbacks
+     * will be unregistered and the behaviour of the would_log() function
+     * sanitized.
+     */
+    void setTestingMode(bool testMode);
+
 private:
     // constructor
     logstream();