Simplify tests by figuring out fixture dir automatically

We always use the same directory for terraform fixtures, so it's quite
easy to figure out its path from a pytest fixture by inspecting the
stack. This commit implements this functionality and decreases the
amount of boilerplate needed to write a test.

(Ported from fast)

tests/modules/cloud_function/test_plan.py

@@ -12,17 +12,12 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-
-import os
 import pytest
 
 
-FIXTURES_DIR = os.path.join(os.path.dirname(__file__), 'fixture')
-
-
 @pytest.fixture
 def resources(plan_runner):
-  _, resources = plan_runner(FIXTURES_DIR)
+  _, resources = plan_runner()
   return resources