Update casper testing documentation.

This adds a note that waitFor functions cannot be chained together if
the wait can cause race condition.

docs/testing.md

@@ -244,6 +244,23 @@ for writing Casper tests in addition to the debugging notes below:
     <http://docs.casperjs.org/en/latest/modules/casper.html#waitforselector>
     and the various assert statements available are documented here:
     <http://docs.casperjs.org/en/latest/modules/tester.html#the-tester-prototype>
+
+- The 'waitFor' style functions (waitForSelector, etc.) cannot be
+    chained together in certain conditions without creating race
+    conditions where the test may fail nondeterministically. For
+    example, don't do this:
+
+        casper.waitForSelector('tag 1');
+        casper.waitForSelector('tag 2');
+
+    Instead, if you want to avoid race condition, wrap the second
+    `waitFor` in a `then` function like this:
+
+        casper.waitForSelector('tag 1');
+        casper.then(function () {
+            casper.waitForSelector('tag 2');
+        });
+
 -   Casper uses CSS3 selectors; you can often save time by testing and
     debugging your selectors on the relevant page of the Zulip
     development app in the Chrome JavaScript console by using e.g.