[FIX] Verify N98_MAGERUN_TEST_MAGENTO_ROOT in Testcase
[FIX] Verify N98_MAGERUN_TEST_MAGENTO_ROOT in Testcase

Fail early and with the information if it is not a directory or realpath
fails.

tkn98/n98-magerun,pocallaghan/n98-magerun,therouv/n98-magerun,pocallaghan/n98-magerun,therouv/n98-magerun,tkn98/n98-magerun,pocallaghan/n98-magerun,tkn98/n98-magerun