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