Fix test certificates module hanging issue (#427)

* Fix test certificates module hanging issue

When executing the lemur/tests/test_certificates.py module's tests, all
tests are executed, but the test process appears to hang and never
completes with the display of the results for the tests.

The hanging issue is traced to the two test methods:
test_import(logged_in_user) and test_upload(logged_in_user). The issue
has to do with the test methods' using the logged_in_user(app) fixture from
the conftest.py module as the method parameter.

The test methods at issue require the session, db, and app fixtures to
be initialized for the tests to complete successfully. The
logged_in_user() fixture only initializes the app fixture. Updating the
test_import() and test_upload() methods parameters to be the "session"
fixture fixes the hanging issue and the tests complete successfully.

This is the command being used to execute the tests...
$ py.test -s -v lemur/tests/test_certificates.py

* Update fix for test certificates hanging issue

Based on feedback from the original pull request for this fix, added the
session fixture to the logged_in_user fixture and reverted the
test_import() and test_upload() methods to use the logged_in_user
(instead of the session fixture).

lemur/tests/conftest.py

@@ -141,7 +141,7 @@ def issuer_plugin():
 
 
 @pytest.yield_fixture(scope="function")
-def logged_in_user(app):
+def logged_in_user(session, app):
     with app.test_request_context():
         identity_changed.send(current_app._get_current_object(), identity=Identity(1))
         yield