Use glance.context.RequestContext in tests

There were some recent changes to oslo.policy that include specific
type checks of the ``creds`` parameter passed to the ``enforce()``
method:

  https://review.openstack.org/#/c/578995/

The above change allows consuming projects to pass instances
of oslo_context.context.RequestContext to the policy enforcer, as
opposed to building a dictionary instance from scratch multiple
different ways across projects.

The glance unit tests were failing with the new version of
oslo.policy (1.38.0) because they were passing in a Mock instance
which failed the new, more strict, type check.

This commit converts the setUp class of the tests to use an instance
of glance.context.RequestContext, which subclasses RequestContext
from oslo.context. It also sets the user_id and project_id attributes
of the context object to override authorization checks.

This should allow glance to pass with newer versions of oslo.policy.

Change-Id: I0a69bc9565d57fd6ad8484abc5fce0e8dd45f9f2
Closes-Bug: 1781976

glance/tests/unit/async/flows/test_import.py

@@ -31,6 +31,7 @@ from glance.async import utils as async_utils
 from glance.common.scripts.image_import import main as image_import
 from glance.common.scripts import utils as script_utils
 from glance.common import utils
+from glance import context
 from glance import domain
 from glance import gateway
 import glance.tests.utils as test_utils
@@ -63,7 +64,9 @@ class TestImportTask(test_utils.BaseTestCase):
         utils.safe_mkdirs(self.work_dir)
         self.config(work_dir=self.work_dir, group='task')
 
-        self.context = mock.MagicMock()
+        self.context = context.RequestContext(
+            user_id=TENANT1, project_id=TENANT1, overwrite=False
+        )
         self.img_repo = mock.MagicMock()
         self.task_repo = mock.MagicMock()