Cert: Add virtualenv setup for cert tests

* Add cert/set_up_virtualenv.sh for cert tests so that:
  1) We can use common IDEs that work with virtualenv to edit GD cert
     tests and libraries
  2) We no longer have to use PYTHONPATH when running tests
  3) Explicitly list dependencies in virtualenv setup to make the setup
     more Android independent as it really just need that two ZIP files
* Fixed some import directives as gd/ should always be the import root
  event when importing from the same directory

Bug: 148147248
Test: set up virtual env and run tests
Change-Id: Ie5b42dc87f2ff67910de085e96cf616b257b0b7f