Matrix: capture stdout for print() test

I want to switch the print() function back to use printf()
such that it's fully compatible with all (embedded) platforms.

To still test the print function I'm capturing stdout into a file
such that the print() function can stay unchanged and the result of
the printf()s can be evaluated.

matrix/Matrix.hpp

@@ -350,13 +350,13 @@ public:
         }
     }
 
-    void print(FILE *stream = stdout) const
+    void print() const
     {
         // element: tab, point, 8 digits, 4 scientific notation chars; row: newline; string: \0 end
         static const size_t n = 15*N*M + M + 1;
         char * buf = new char[n];
         write_string(buf, n);
-        fprintf(stream, "%s\n", buf);
+        printf("%s\n", buf);
         delete[] buf;
     }