Split the read-receipt tests into logical units (#11649)

* Split the read-receipt tests into logical units

* Move read-receipt docs into a readme file

* Provide doc comments for exported functions in read-receipt tests

cypress/e2e/read-receipts/readme.md

@@ -0,0 +1,20 @@
+# High Level Read Receipt Tests
+
+Tips for writing these tests:
+
+-   Break up your tests into the smallest test case possible. The purpose of
+    these tests is to understand hard-to-find bugs, so small tests are necessary.
+    We know that Cypress recommends combining tests together for performance, but
+    that will frustrate our goals here. (We will need to find a different way to
+    reduce CI time.)
+
+-   Try to assert something after every action, to make sure it has completed.
+    E.g.:
+    markAsRead(room2);
+    assertRead(room2);
+    You should especially follow this rule if you are jumping to a different
+    room or similar straight afterwards.
+
+-   Use assertStillRead() if you are asserting something is read when it was
+    also read before. This waits a little while to make sure you're not getting a
+    false positive.