Tests not correctly linked against *_debug.so libraries

When the tests are built, they are linked against $(top_builddir)/lib/libnorns_debug.la and $(top_builddir)/lib/libnornsctl_debug.la which correspond to actual shared objects that will eventually be installed. Nevertheless, since tests should be run prior to installing, this means that, as it is now, tests won't run due to missing dependencies unless the libraries have already been installed to a standard location or the LD_LIBRARY_PATH environment variable has been set accordingly.