node tests: Introduce message_store.create_mock_message() helper.

Previously, it was tedious to create actual message
objects in message_store for use in node tests.
This was mainly because, `add_message_metadata`
in message_store has many dependencies and
validation checks. Since it was difficult to create
actual message objects, many tests just mocked
the `message_store.get()` method to return the desired
message.

This commit adds a new helper method (`create_mock_message`)
to message_store, for use in node tests. This just stores
the object passed to it in the `stores_messages` map,
without any validation. We do not add any
default fields to the message object before saving
it from this helper, because doing so would decrease
the utility of this helper, and, if a test
depends on some field having a particular value,
then it would be better to just pass the field: value
pair from the test itself, for readability, rather
than relying on the helper to add the field for us.

This helper allows us to write deeper tests.

This commit also replaces some instances of mocking
`message_store.get()` to use this new helper method.

static/js/message_store.js

@@ -33,6 +33,12 @@ export function user_ids() {
     return Array.from(message_user_ids);
 }
 
+export function create_mock_message(message) {
+    // For use in tests only. `id` is a required field,
+    // everything else is optional, as required in the test.
+    stored_messages.set(message.id, message);
+}
+
 export function get(message_id) {
     if (message_id === undefined || message_id === null) {
         blueslip.error("message_store.get got bad value: " + message_id);