fix(tests): Modernize unit tests of Notifications (#366)

Replaces PR #326. Closes #212.

## Changes

- all notif tests are now pure Mocha tests. => no more wrappers
- all async functions now return Promises => no more callbacks
- each test cleans up the db before running => no more repeated "clean-up" tests

app/models/notif.js

@@ -198,7 +198,7 @@ exports.clearUserNotifsForPost = function (uId, pId) {
   );
 };
 
-exports.clearUserNotifs = function (uId) {
+exports.clearUserNotifs = function (uId, cb) {
   if (!uId) return;
   db['notif'].find({ uId: uId }, { uId: 1 }, { limit: 1000 }, function (
     err,
@@ -215,9 +215,12 @@ exports.clearUserNotifs = function (uId) {
           db['notif'].update(
             { uId: uId },
             { $pull: { uId: uId } },
-            { multi: true, w: 0 }
+            { multi: true, w: 0 },
+            () => {
+              invalidateUserNotifsCache(uId);
+              cb && cb();
+            }
           );
-          invalidateUserNotifsCache(uId);
         }
       );
     }
@@ -292,7 +295,7 @@ exports.html = function (uId, html, href, img) {
 
 // specific notification methods
 
-exports.love = function (loverUid, post) {
+exports.love = function (loverUid, post, callback) {
   var user = mongodb.usernames['' + loverUid];
   var author = mongodb.usernames['' + post.uId];
   if (!user || !author) return;
@@ -309,7 +312,8 @@ exports.love = function (loverUid, post) {
       $push: { lov: loverUid },
       $inc: { n: 1 },
     },
-    { upsert: true, w: 0 }
+    { upsert: true, w: 0 },
+    callback
   );
   invalidateUserNotifsCache(post.uId); // author will be invalidated later by clearUserNotifsForPost()
   notifEmails.sendLike(user, post, author);