test: add one gl2d mock to export test list

Author: etpinard

test/image/export_test.js

@@ -18,7 +18,9 @@ var test = require('tape');
 var FORMATS = ['svg', 'pdf', 'eps'];
 
 // non-exhaustive list of mocks to test
-var DEFAULT_LIST = ['0', 'geo_first', 'gl3d_z-range', 'text_export', 'layout_image'];
+var DEFAULT_LIST = [
+    '0', 'geo_first', 'gl3d_z-range', 'text_export', 'layout_image', 'gl2d_12'
+];
 
 // return dimensions [in px]
 var WIDTH = 700;