Reset heartbeat timer when test files are served.

The heartbeat timer is only reset when YUI Test reports
a test pass or fail event. If a test fails to run any YUI Tests
before the heartbeat timeout of 45 seconds, then future tests
will immediately hit this timeout because the timeout is not
cleared when a new test is requested.

This fix causes the timer to reset when test-related files
are served, which includes requesting a new test.

Credit to John Traver <jtraver@yahoo-inc.com> for
reporting this issue.

lib/hub/batch.js

@@ -201,14 +201,16 @@ Batch.prototype.getFile = function (filename, cb) {
 };
 
 Batch.prototype.handleFileRequest = function (server, agentId, filename) {
-    var batch = this;
+    var agent,
+        batch = this;
+
+    if (agentId) {
+        agent = batch.agents[agentId];
+    }
+
     this.getFile(filename, function (err, buffer) {
         if (err) {
-            var agent,
-                fileInBatch;
-            if (agentId) {
-                agent = batch.agents[agentId];
-            }
+            var fileInBatch;
             if (agent) {
                 // If this file is in the current test batch,
                 // redirect to the next test.
@@ -238,6 +240,12 @@ Batch.prototype.handleFileRequest = function (server, agentId, filename) {
             }
             return;
         }
+
+        if (agent) {
+            batch.debug("Recording ping for agentId =", agentId, "for file =", filename);
+            agent.ping();
+        }
+
         batch.testServer.serve(server, filename, buffer);
     });
 };