Add role assignment test coverage for system admin

This commit adds role assignment test coverage for users who have the
admin role assigned on the system.

Subsequent patches will:

  - add functionality for domain readers
  - add functionality for domain members
  - add functionality for domain admins
  - add functionality for project readers
  - add functionality for project members
  - add functionality for project admins
  - remove the obsolete policies from policy.v3cloudsample.json

Change-Id: If0d418a7117623b3bfe11b8e23781d02ac1debf0
Partial-Bug: 1750673
Closes-Bug: 1816833

keystone/tests/unit/protection/v3/test_assignment.py

@@ -731,3 +731,30 @@ class SystemMemberTests(base_classes.TestCaseWithBootstrap,
             r = c.post('/v3/auth/tokens', json=auth)
             self.token_id = r.headers['X-Subject-Token']
             self.headers = {'X-Auth-Token': self.token_id}
+
+
+class SystemAdminTests(base_classes.TestCaseWithBootstrap,
+                       common_auth.AuthTestMixin,
+                       _AssignmentTestUtilities,
+                       _SystemUserTests):
+
+    def setUp(self):
+        super(SystemAdminTests, self).setUp()
+        self.loadapp()
+        self.useFixture(ksfixtures.Policy(self.config_fixture))
+        self.config_fixture.config(group='oslo_policy', enforce_scope=True)
+
+        self.user_id = self.bootstrapper.admin_user_id
+        self.expected = []
+
+        auth = self.build_authentication_request(
+            user_id=self.user_id, password=self.bootstrapper.admin_password,
+            system=True
+        )
+
+        # Grab a token using the persona we're testing and prepare headers
+        # for requests we'll be making in the tests.
+        with self.test_client() as c:
+            r = c.post('/v3/auth/tokens', json=auth)
+            self.token_id = r.headers['X-Subject-Token']
+            self.headers = {'X-Auth-Token': self.token_id}