Add irnetproxy tests into the existing test runners

 * make check-pylint now runs on irnetproxy and irnetproxy_test
 * make check-nosetests includes the irnetproxy_test module
 * New python dependency 'mock' added to the .tests.yml file

Note, mock is pip installed because the version in the ubuntu
deb repositories is too old (doesn't support return_value= argument)