set api.extensions logging to ERROR in unit tests

neutron.api.extensions generates a lot of output in a normal
unit test run. It's responsible for > 80% of the lines in the
subunit_log.txt. Logging each file load 10k times slows everything
down, takes up valuable memory, and doesn't really help with testing.

Instead we can set this to ERROR level in a fixture to ignore most
of that logging and help make the unit tests run faster.

Change-Id: I86b41f20342b4080859ee036220ddffa200f0562

neutron/tests/base.py

@@ -96,6 +96,15 @@ class BaseTestCase(testtools.TestCase):
                 nuke_handlers=capture_logs,
             ))
 
+        # suppress all but errors here
+        self.useFixture(
+            fixtures.FakeLogger(
+                name='neutron.api.extensions',
+                format=LOG_FORMAT,
+                level=logging.ERROR,
+                nuke_handlers=capture_logs,
+            ))
+
         test_timeout = int(os.environ.get('OS_TEST_TIMEOUT', 0))
         if test_timeout == -1:
             test_timeout = 0