API:

- replaced BKE_copy_images with BKE_export_image, now it handles only one image at a time, this is better since for exporters it is 
easier to export one image at a time writing new image path to a file
- exposing BKE_export_image in RNA as Image.export, interestingly, RNA allowed me to define a function with PROP_STRING return type 
although it doesn't free memory, will fix that in the next commit
- removed bpy.util.copy_images

Unit tests:
- re-wrote a test for BKE_export_image, it's more compact now
- moved unit tests to the creator module to avoid another executable, now running tests with `blender --test`
- as before, unit tests are built only if WITH_BF_UNIT_TEST is non 0

source/blender/python/intern/bpy_interface.c

@@ -93,7 +93,7 @@ void BPY_update_modules( void )
 	PyObject *mod= PyImport_ImportModuleLevel("bpy", NULL, NULL, NULL, 0);
 	PyModule_AddObject( mod, "data", BPY_rna_module() );
 	PyModule_AddObject( mod, "types", BPY_rna_types() );
-	PyModule_AddObject( mod, "util", BPY_util_module() );
+	/* PyModule_AddObject( mod, "util", BPY_util_module() ); */
 
 	/* XXX this will move to bpy.util */
 	PyModule_AddObject( mod, "sys", BPY_sys_module() );