You can use provided Makefile in order to build testdays. By default, it'll use buildroot equal to your current OS. You can override this behaviour by sepcifiyng variables for Make manually.
To build testdays:
make mockbuild
To build testdays for Fedora 27:
make BUILDTARGET=fedora-27-x86_64 TARGETDIST=fc27 mocksrpm
To build testdays for CentOS 7:
make BUILDTARGET=epel-7-x86_64 TARGETDIST=el7 mocksrpm