fix flaky test failure from importing webutil.testutil_appengine

eg https://circleci.com/gh/snarfed/bridgy-fed/164
```
testutil_appengine (unittest.loader.ModuleImportFailure) ... ERROR
...
======================================================================
ERROR: testutil_appengine (unittest.loader.ModuleImportFailure)
----------------------------------------------------------------------
ImportError: Failed to import test module: testutil_appengine
Traceback (most recent call last):
  File "/usr/lib/python2.7/unittest/loader.py", line 254, in _find_tests
    module = self._get_module_from_name(name)
  File "/usr/lib/python2.7/unittest/loader.py", line 232, in _get_module_from_name
    __import__(name)
  File "/tmp/oauth-dropins/oauth_dropins/webutil/testutil_appengine.py", line 4, in <module>
    from google.appengine.datastore import datastore_stub_util
  File "/usr/lib/google-cloud-sdk/platform/google_appengine/google/appengine/datastore/datastore_stub_util.py", line 69, in <module>
    from google.appengine.datastore import datastore_stub_index
  File "/usr/lib/google-cloud-sdk/platform/google_appengine/google/appengine/datastore/datastore_stub_index.py", line 45, in <module>
    import yaml
ImportError: No module named yaml
```

.circleci/config.yml

@@ -38,8 +38,7 @@ jobs:
             ln -s $VIRTUAL_ENV local  # we use app engine's vendor module to point here
             pip install coverage coveralls  # for https://coveralls.io/
 
-            . local/bin/activate
-            python2 -m coverage run --source=. --omit=appengine_config.py,local/\*,tests/\* -m unittest discover -v
+            python2 -m coverage run --source=. --omit=appengine_config.py,local/\*,oauth-dropins/\*,tests/\* -m unittest discover -v
             python2 -m coverage html -d /tmp/coverage_html
 
             # send coverage data to coveralls