filetemplates plugin: Make testfiletemplates depend on kdevfiletemplates

There is currently no dependency between both targets, however some of
the files built by both testfiletemplates and kdevfiletemplates (such as
classidentifierpage.cpp) depend on ui files that only the latter target
generates. This means that if one does make testfiletemplates the build
fails like this:

/tmp/kdevplatform/plugins/filetemplates/classidentifierpage.cpp:23:25:
fatal error: ui_newclass.h: No such file or directory

Differential Revision: https://phabricator.kde.org/D1160