If there's no test dir set, use "config.tests" as default value, and
perform the check for the existence of CMakeLists.txt in
configurejson2cmake, not the configure.cmake file.
Change-Id: I8d3be8f2b68767c7592665c374022353c2dc6c57
Reviewed-by: Leander Beernaert <leander.beernaert@qt.io>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>