Use system /tmp for temp files when testing actionpack

https://github.com/rails/rails/commit/c64bff2c87ebf363703c63ecd4a96d56a1a78364 added support for running the actionpack tests in parallel, and DRb tries to.

However it introduced https://github.com/rails/rails-dev-box/issues/76 since one cannot connect to a socket file that's inside a Vagrant synced folder due to security restrictions.

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