Add thread list endpoint to the new discussion API

This is an initial implementation that only allows retrieval of all
threads for a course and only returns an easily computed subset of the
fields that are needed, in order to keep this change from getting too
large.

JIRA: MA-641

lms/djangoapps/discussion_api/tests/utils.py

@@ -0,0 +1,30 @@
+"""
+Discussion API test utilities
+"""
+import json
+
+import httpretty
+
+
+class CommentsServiceMockMixin(object):
+    """Mixin with utility methods for mocking the comments service"""
+    def register_get_threads_response(self, threads, page, num_pages):
+        """Register a mock response for GET on the CS thread list endpoint"""
+        httpretty.register_uri(
+            httpretty.GET,
+            "http://localhost:4567/api/v1/threads",
+            body=json.dumps({
+                "collection": threads,
+                "page": page,
+                "num_pages": num_pages,
+            }),
+            status=200
+        )
+
+    def assert_last_query_params(self, expected_params):
+        """
+        Assert that the last mock request had the expected query parameters
+        """
+        actual_params = dict(httpretty.last_request().querystring)
+        actual_params.pop("request_id")  # request_id is random
+        self.assertEqual(actual_params, expected_params)