Update testing helpers

I was getting the deprecation warning:

    Assigning the current_user this way has been deprecated. You should instead use the sign_in method.

After [removing all instances of `current_user` assignment from the app](https://github.com/rubygems/rubygems.org/commit/9272defb7a78ebba85959f7245b606c011687780), I was still receiving these warnings, so I added a bit more information to the warning message, specifically, the file and line number from which the warning was being generated.

It turns out that the warnings were coming from inside the `clearance` codebase, in the testing helpers.

I’ve fixed the deprecation warnings and left in the improved warning information. I’ve committed these two changes separately so that you may cherry-pick just the fix if, for whatever reason, you don’t wanted the more detailed warning messages.