Use system /tmp when testing actionpack

https://github.com/rails/rails/commit/c64bff2c87ebf363703c63ecd4a96d56a1a78364
added support and enabled parallel execution of the actionpack tests.

However it introduced https://github.com/rails/rails/commit/c64bff2c87ebf363703c63ecd4a96d56a1a78364
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 https://github.com/rails/rails/commit/c64bff2c87ebf363703c63ecd4a96d56a1a78364
Merge pull request #16644 from Agis-/drb-tests-actionpack-vagrant

Use system /tmp for temp files when testing actionpack
After rack 1.6.0, by default, the rack will also out put the log which is not intent in rails.

Such change will also close the quiet_assets [#36](https://github.com/evrone/quiet_assets/issues/36) issue.