CI is failing after herokuapp.com default domain name change

[edmorley]

CI on main is currently failing with:

Failures:

  1) Compojure on jdk-1.8 deploys clojure-minimal successfully
     Failure/Error: expect(http_get(app)).to include('["Hello" :from Heroku]')

     Excon::Error::NotFound:
       Expected(200) <=> Actual(404 Not Found)

eg: https://github.com/heroku/heroku-buildpack-clojure/actions/runs/5531841256/jobs/10093073350?pr=142#step:5:477

This is due to: https://devcenter.heroku.com/changelog-items/2640

And means needing to update this helper: https://github.com/heroku/heroku-buildpack-clojure/blob/2a44ec32a77ea4e3b9fd4ffc6bdd335fafef3c4e/test/spec/spec_helper.rb#L60-L64

Similar to: https://github.com/heroku/heroku-buildpack-nodejs/pull/1110/files

This blocks being able to merge #142.

cc @Malax