Ignore SAWarnings for "Evaluating non-mapped column expression"

These warnings come from oslo.db code (tracked with bug 1814199)
so there isn't much nova can do about that right now, outside of
monkey patching oslo.db which is a bad idea.

Let's ignore the warning until the bug in oslo.db is fixed to
avoid blowing up our unit/functional test console output logs
which in turn is intermittently triggering subunit.parser failures.

Change-Id: Ifdfeadae8b348d788de2cd665544015366271d66
Related-Bug: #1813147

nova/tests/fixtures.py

@@ -37,6 +37,7 @@ from oslo_privsep import daemon as privsep_daemon
 from oslo_utils.fixture import uuidsentinel
 from oslo_utils import uuidutils
 from requests import adapters
+from sqlalchemy import exc as sqla_exc
 from wsgi_intercept import interceptor
 
 from nova.api.openstack.compute import tenant_networks
@@ -836,6 +837,12 @@ class WarningsFixture(fixtures.Fixture):
                     "._Default object at ",
             category=UserWarning)
 
+        # TODO(mriedem): Change (or remove) this SAWarning to an error once
+        # https://bugs.launchpad.net/oslo.db/+bug/1814199 is fixed.
+        warnings.filterwarnings(
+            'ignore', message='Evaluating non-mapped column expression',
+            category=sqla_exc.SAWarning)
+
         self.addCleanup(warnings.resetwarnings)