test(collector): temporarily bring back the mock agent Server header

We have a couple of tests that install the latest released package from
npm, and those still look for the Server header, until the previous
commit has been released as a package. After that, we can remove the
Server header from the mock agent.

packages/collector/test/apps/agentStub.js

@@ -50,6 +50,11 @@ app.use(
   })
 );
 
+app.use((req, res, next) => {
+  res.set('server', 'Instana Agent');
+  next();
+});
+
 app.get('/', (req, res) => {
   res.send('OK');
 });