Use system /tmp when testing actionpack

c64bff2c87
added support and enabled parallel execution of the actionpack tests.

However it introduced c64bff2c87
since one cannot connect to a socket file that's inside a Vagrant synced folder
due to security restrictions, and DRb tries to.

Also rename the temporary files to make it obvious that they're rails-related,
since now they're placed outside the project's directory.

Fixes c64bff2c87

actionpack/test/abstract_unit.rb

@@ -4,8 +4,6 @@
 $:.unshift(File.dirname(__FILE__) + '/fixtures/helpers')
 $:.unshift(File.dirname(__FILE__) + '/fixtures/alternate_helpers')
 
-ENV['TMPDIR'] = File.join(File.dirname(__FILE__), 'tmp')
-
 require 'active_support/core_ext/kernel/reporting'
 
 # These are the normal settings that will be set up by Railties
@@ -466,7 +464,7 @@ def pop; @queue.pop; end
   def initialize size
     @size  = size
     @queue = Server.new
-    file   = File.join Dir.tmpdir, Dir::Tmpname.make_tmpname('tests', 'fd')
+    file   = File.join Dir.tmpdir, Dir::Tmpname.make_tmpname('rails-tests', 'fd')
     @url   = "drbunix://#{file}"
     @pool  = nil
     DRb.start_service @url, @queue