refactor: replace usages of XModuleMixin.system with XBlock.runtime

xmodule/tests/test_capa_block.py

@@ -1734,7 +1734,7 @@ class ProblemBlockTest(unittest.TestCase):  # lint-amnesty, pylint: disable=miss
         error_msg = "Superterrible error happened: ☠"
         block.lcp.get_html = Mock(side_effect=Exception(error_msg))
 
-        block.system.is_author_mode = True
+        block.runtime.is_author_mode = True
 
         # Try to render the block with the author mode turned on
         html = block.get_problem_html()

xmodule/tests/test_split_test_block.py

@@ -124,7 +124,7 @@ class SplitTestBlockTest(XModuleXmlImportTest, PartitionTestCase):
         # view, since mock services exist and the rendering code will not short-circuit.
         mocked_modulestore = Mock()
         mocked_modulestore.get_course.return_value = self.course
-        self.split_test_block.system.modulestore = mocked_modulestore
+        self.split_test_block.runtime.modulestore = mocked_modulestore
 
 
 @ddt.ddt
@@ -239,7 +239,7 @@ class SplitTestBlockStudioTest(SplitTestBlockTest):
         mocked_course = Mock(advanced_modules=['split_test'])
         mocked_modulestore = Mock()
         mocked_modulestore.get_course.return_value = mocked_course
-        self.split_test_block.system.modulestore = mocked_modulestore
+        self.split_test_block.runtime.modulestore = mocked_modulestore
 
         self.split_test_block.user_partitions = [
             UserPartition(0, 'first_partition', 'First Partition', [Group("0", 'alpha'), Group("1", 'beta')])