Fixed plugin_load_option failure, when example storage

engine is not available. We need to add loose prefix
to example load option.

mysql-test/t/plugin_load_option-master.opt

 $EXAMPLE_PLUGIN_OPT
 $EXAMPLE_PLUGIN_LOAD
---plugin-example=FORCE_PLUS_PERMANENT
+--loose-plugin-example=FORCE_PLUS_PERMANENT