Add last test launch time in the new /api/runs route

lib/server/datastores/runsQueue.js

@@ -6,7 +6,7 @@ function RunsQueue() {
     'use strict';
 
     var queue = [];
-
+    var lastTestTimestamp = 0;
 
     this.push = function(runId) {
         var deferred = Q.defer();
@@ -21,6 +21,7 @@ function RunsQueue() {
                 runId: runId
             });
             
+            lastTestTimestamp = Date.now();
             deferred.resolve();
 
         } else {
@@ -31,6 +32,7 @@ function RunsQueue() {
                     deferred.notify(position);
                 },
                 itIsTimeCallback: function() {
+                    lastTestTimestamp = Date.now();
                     deferred.resolve();
                 }
             });
@@ -78,6 +80,11 @@ function RunsQueue() {
     this.length = function() {
         return queue.length;
     };
+
+    // Returns the number of seconds since the last test was launched
+    this.timeSinceLastTestStarted = function() {
+        return Math.round((Date.now() - lastTestTimestamp) / 1000);
+    };
 }
 
 module.exports = RunsQueue;