This is a prep commit to extract out the test cases
for 'test_is_same_server_message_link' in hash_util.test.js
as a test fixture.
This will help in reusing the same test cases in the node and
backend tests.
Also, it will help in making sure that the logic for
'is_same_server_message_link' is in sync, in both the
web client and server code.
Prakhar Pratyush