New search paths:

• (GenericDataLocation)/help
• a custom path (absolute path), which can be defined during compile time by setting KDOCTOOLS_CUSTOM_DOC_PATH

Also, make sure that the first item looks into
KDE_INSTALL_DOCSBUNDLEDIR anyway.

The addition of <datadir>/help (usually /usr/share/help) paves
the way for using that directory as new default value for
KDE_INSTALL_DOCSBUNDLEDIR.

The test passes and the list of directories contains
the expected entries.