runner: replace parallel loading of resources with sequential one

The runner would load all the client resource files in parallel, causing errors
intermittently due to some resources depending on other ones.
This behavior was incorrect.

Runner to uses `resolveSequentially` that ensure that each resource is loaded in
order.

lib/runner.js

 const webdriverio = require('webdriverio');
 
+// Utility function to resolve a list of promise-based function calls
+// on a given element list sequentially
+function resolveSequentially(f, elements) {
+  return elements.reduce(
+    (acc, curr) => acc.then(() => f(curr)).catch(err => console.error(err)),
+    new Promise(resolve => resolve()));
+}
+
 module.exports = config => ({
   run: (controllerFilesList, server, room, clientConfig) => {
     const client = webdriverio.remote(config.driver);
     return client.init()
       .url(server + '/' + room)
-      .then(
-        () => Promise.all(controllerFilesList.map(
-          f => client.execute(f, room, clientConfig))))
+      .then(() => resolveSequentially(f => client.execute(f, room, clientConfig),
+                                      controllerFilesList))
       .then(() => client.execute(() => {
         /* eslint-disable no-undef */
         robot.start();