node-tests: Update references to "private message" and "PM".

Updates any use of "private message" and "PM" in the `web/tests`
directory to instead use "direct messages".

web/tests/user_topics.test.js

@@ -50,7 +50,7 @@ function test(label, f) {
 }
 
 test("edge_cases", () => {
-    // private messages
+    // direct messages
     assert.ok(!user_topics.is_topic_muted(undefined, undefined));
 });