Fix test failures when run consecutively

Problem:
Running tests twice causes failures

Solution:
Remove the memoize cache before test runs

This would fail if run twice:
./manage.py test --keepdb jetstream.test_tas_api.TestJetstream.test_validate_account

It would fail because the cassettes did not reflect the correct number of
plays, i.e. api calls didn't happen like we expected. Normally cassettes store
the results of a first http call, and intercept and respond to all future calls with the
results of the first. We assert that the cassette was called a certain number
of times. However, django-memoize intercepts the functions that would even
make these http calls, so the cassettes were under-reporting the number of
playbacks.

jetstream/test_tas_api.py

@@ -1,5 +1,5 @@
 import json
-
+import memoize
 import freezegun
 import vcr
 from django.test import TestCase, override_settings, modify_settings
@@ -23,6 +23,14 @@
 class TestJetstream(TestCase):
     """Tests for Jetstream allocation source API"""
 
+    def setUp(self):
+        # Because we use memoize to cache the tacc api, calling tacc_api_get
+        # doesn't necessarily trigger an http request. This means that a
+        # cassette will not necessarily be played. In order to test cassette
+        # playback, we just need to clear the memoize cache
+        from jetstream.tas_api import tacc_api_get
+        memoize.delete_memoized(tacc_api_get)
+
     @my_vcr.use_cassette()
     def test_validate_account(self, cassette):
         """Test for a valid account based on the business logic assigned by Jetstream"""