Show only the first difference when images differ in the tests

The code was supposed to do this, but didn't, as "break" only broke from
the inner loop, but not the outer one, so replace it with "return".

tests/testimage.h

@@ -62,7 +62,9 @@ public:
                                     "expected 0x%06x",
                                     x, y, *d2, *d1
                                );
-                    break;
+
+                    // Don't show all mismatches, there may be too many of them.
+                    return false;
                 }
 
                 ++d1;
@@ -70,6 +72,10 @@ public:
             }
         }
 
+        // We should never get here as we know that the images are different
+        // and so should have returned from inside the loop above.
+        wxFAIL_MSG("unreachable");
+
         return false;
     }