tests: Decouple test_retention and test_reactions.

We generally want to avoid having two sibling test
suites depend on each other, unless there's a real
compelling reason to share code.  (And if there is
code to share, we can usually promote it to either
test_helpers or ZulipTestCase, as I did here.)

This commit is also prep for the next commit, where
I try to simplify all of the helpers in EmojiReactionBase.

Especially now that we have f-strings, it is usually
better to just call api_post explicitly than to
obscure the mechanism with thin wrappers around
api_post.  Our url schemes are pretty stable, so it's
unlikely that the helpers are actually gonna prevent
future busywork.

zerver/lib/test_helpers.py

@@ -591,3 +591,10 @@ def create_dummy_file(filename: str) -> str:
     with open(filepath, 'w') as f:
         f.write('zulip!')
     return filepath
+
+def zulip_reaction_info() -> Dict[str, str]:
+    return dict(
+        emoji_name='zulip',
+        emoji_code='zulip',
+        reaction_type='zulip_extra_emoji',
+    )