Reimplement Rack::Auth::Digest in tests

[pcai]

background: https://www.rfc-editor.org/rfc/rfc2617

rack 3.0 deprecates the server-side implementation of HTTP digest authentication (Rack::Auth::Digest) and 3.1 removes it. the old implementation is here:

https://github.com/rack/rack/blob/3-0-stable/lib/rack/auth/digest.rb

Our integration tests relied on this and start failing when bundled with rack 3.1 because the class is missing.

Fortunately, the implementation is not used in the library, only tests, so we'll just skip the tests for now. This will allow us to loosen the deps so that httpi can be used with rack 3.1