Add regression test for circular require across load orders

[lineoffligbot]

Adds a regression test that asserts no circular require warning is emitted for any load order of the gem's entry points.

Refs #66 (warning under require "http/cookie_jar"; require "http/cookie", which is what require "http" triggers via http/session preceding http/response). This test also covers the scenarios fixed in #65.

The test currently fails on scenario 4 (http/cookie_jar then http/cookie) on master, demonstrating #66. Opening as a draft — happy to adjust style/location, and I can follow up with a fix PR once you've confirmed a preferred direction in #66.

Implementation notes:

• Subprocesses are required because circular-require warnings only fire on first load per process.
• Bundler.with_unbundled_env scrubs bundle exec's env injections so the child process doesn't pre-activate the installed gem (which masks the warning).