Enhancement in tests runner

Now we set a PYTHONPATH variable instead of hacking inside python code

runtest.sh

     mkdir $logdir
 fi
 
-PYTHON='env python3'
+export PYTHONPATH="$PYTHONPATH:$(pwd)"
+PYTHON="env python3"
 ret_status=0
 
 $PYTHON ./nocontext_tests.py $logdir $@ || ret_status=1

runtest_context.sh

 #
 # https://docs.python.org/3.4/library/unittest.html
 #
+export PYTHONPATH="$(pwd)/$(dirname $0)/:$PYTHONPATH"
+
 if test ! -f lodel/buildconf.py
 then
 	echo "You have to build the project before running the tests"
 	exit 1
 fi
 
-PYTHON='env python3'
+PYTHON="env python3"
 testdir=$(mktemp -td "lodel2_unittest_instance_XXXXXXXX")
 install_model_dir="[@]INSTALLMODEL_DIR[@]"
 if [ ! -d "$install_model_dir" ]