Workaround issue with sprockets-4.0.0.beta

The current sprockets-4.0.0.beta code does not like 'rake assets:clobber' when
this directory is missing.  Depending on the order tests are run, this may or
may not exist and cause an error.  I will follow up with a bug/PR there.

Author: lostapathy

test/test_railtie.rb

@@ -57,6 +57,12 @@ class TestRailtie < TestBoot
   def setup
     require 'sprockets/railtie'
     super
+
+    # sprockets-4.0.0.beta8 does not like 'rake assets:clobber' when this directory does not exist
+    Dir.chdir(app.root) do
+      dir = "tmp/cache/assets/sprockets"
+      FileUtils.mkdir_p(dir)
+    end
   end
 
   def test_defaults_to_compile_assets_with_env_and_manifest_available