Get testrunner path from config file

A little modification which make it possible to use own ``testrunner`` module. Instead of relying on a hardcoded value, it is defined in ``config`` file.

A [hack](https://github.com/revel/revel/blob/df5d88dbece276012b4c7ab45a33a1047c1db108/revel.go#L322) for adding "test" subdirectory to the CodePaths is still there.