Cherry pick the test-cleanup parts of e2826cb.

- look up test courses by id, not name

common/lib/xmodule/xmodule/modulestore/__init__.py

@@ -339,6 +339,12 @@ class ModuleStore(object):
         '''
         raise NotImplementedError
 
+    def get_course(self, course_id):
+        '''
+        Look for a specific course id.  Returns the course descriptor, or None if not found.
+        '''
+        raise NotImplementedError
+
     def get_parent_locations(self, location):
         '''Find all locations that are the parents of this location.  Needed
         for path_to_location().
@@ -399,3 +405,10 @@ class ModuleStoreBase(ModuleStore):
 
         errorlog = self._get_errorlog(location)
         return errorlog.errors
+
+    def get_course(self, course_id):
+        """Default impl--linear search through course list"""
+        for c in self.get_courses():
+            if c.id == course_id:
+                return c
+        return None