testing: Flush API_KEYS before every test.

This makes our test performance for each test more consistent.

zerver/lib/test_classes.py

@@ -64,6 +64,11 @@ import six
 
 API_KEYS = {} # type: Dict[Text, Text]
 
+def flush_caches_for_testing():
+    # type: () -> None
+    global API_KEYS
+    API_KEYS = {}
+
 class UploadSerializeMixin(SerializeMixin):
     """
     We cannot use override_settings to change upload directory because

zerver/lib/test_runner.py

@@ -14,6 +14,7 @@ from django.test.signals import template_rendered
 
 from zerver.lib import test_classes, test_helpers
 from zerver.lib.cache import bounce_key_prefix_for_testing
+from zerver.lib.test_classes import flush_caches_for_testing
 from zerver.lib.sqlalchemy_utils import get_sqlalchemy_connection
 from zerver.lib.test_helpers import (
     get_all_templates, write_instrumentation_reports,
@@ -111,6 +112,7 @@ def run_test(test, result):
     test_name = full_test_name(test)
 
     bounce_key_prefix_for_testing(test_name)
+    flush_caches_for_testing()
 
     if not hasattr(test, "_pre_setup"):
         # test_name is likely of the form unittest.loader.ModuleImportFailure.zerver.tests.test_upload