Fixes the bug with flat layout and identical target names in subprojects.
Without this change directories are not created with subproject prefix
and they can collide.
Remove dead makedirs code in Backend.__init__(), during initialization
of backend build.targets is empty. Create output directories in
Vs2010Backend.generate_projects() instead.
Also use double blank line in run_unittests.py according to
https://www.python.org/dev/peps/pep-0008/#blank-lines.