Mocking does not work on helper

[ScreamingDev]

No "data" is added. Rather do "get_class(Mage::helper(...)) than finding it out by anything else.

\Mage_Core_Model_Config::getHelperClassName will add data and that is used by "mage::helper"

your \EcomDev_PHPUnit_Test_Case::getGroupedClassName and every previous just ignores it and adds no "/data".

So the class will be "Acme_Extension_Helper" which is obviously wrong.