Ensure unit tests are not built until after src directory

[jwlodek]

Without this change, when cloning the most recent version and compiling with the -j flag, the compilation will fail by trying to build the unit tests directory before the source directories have been built.

[LeandroMartinsdS]

Thank you @jwlodek.

I will merge your changes.
FWIW I'll add the dependency of etc later, to make it possible to use -j when dls-xml-iocbuilder.py is present.