investigate http-compatibility-tests

* https://github.com/apache/incubator-pekko-http/tree/main/http-compatibility-tests
* don't seem to be wired into our sbt setup
* In #354 - one of the files in http-compatibility-tests seems to refer to a class that has been renamed long ago (Akka to Pekko) so the code shouldn't have been able to compile

Start with the http-compatibility-tests directory and the repository's sbt setup. Compare the referenced file and class name with the rename noted in issue #354, then determine whether these tests are included in the build. Done means the compatibility tests are correctly wired and the stale Akka reference is resolved.

Written by the indexing model from the issue text.