[debian] disable tests when building package

For now the test cannot pass because they require cubicweb stack which is not
available in our repository used at build time.

debian/rules

 #!/usr/bin/make -f
 
 export PYBUILD_NAME=cubicweb-postgis
+export PYBUILD_DISABLE=test
 
 %:
 	dh $@ --with python2,python3 --buildsystem=pybuild