Ensure dump folder exists before running LinkedFiles tests

The folder is also created in another test, so it will work the second
time the tests are executed, but it fails the first time.

services/web/test/acceptance/coffee/LinkedFilesTests.coffee

@@ -1,7 +1,9 @@
 async = require "async"
 expect = require("chai").expect
 _ = require 'underscore'
+mkdirp = require "mkdirp"
 
+Settings = require "settings-sharelatex"
 MockFileStoreApi = require './helpers/MockFileStoreApi'
 request = require "./helpers/request"
 User = require "./helpers/User"
@@ -21,8 +23,9 @@ describe "LinkedFiles", ->
 	before (done) ->
 		LinkedUrlProxy.listen 6543, (error) =>
 			return done(error) if error?
-			@owner = new User() 
-			@owner.login done
+			@owner = new User()
+			@owner.login ->
+				mkdirp Settings.path.dumpFolder, done
 
 	describe "creating a URL based linked file", ->
 		before (done) ->
@@ -181,4 +184,4 @@ describe "LinkedFiles", ->
 						done()
 
 		# TODO: Add test for asking for host that return ENOTFOUND
-		# (This will probably end up handled by the proxy)
+		# (This will probably end up handled by the proxy)