add treemap_textposition to list of flaky mocks

... this list will grow eventually.

Author: etpinard

test/image/compare_pixels_test.js

@@ -101,6 +101,7 @@ if(allMock || argv.filter) {
 }
 
 var FLAKY_LIST = [
+    'treemap_textposition'
 ];
 
 console.log('');