review how easy it is to run manual tests

* `sbt test` works fine on my laptop
* the tests in this repo are simpler than the ones in https://github.com/apache/incubator-pekko
* there could be some flaky tests that might cause issues when other people review this module (during a release, for instance)

Start by running `sbt test` in the pekko-http repository and review how the tests are organized. Compare the test setup with the Apache Pekko repository referenced in the issue, then identify which manual-test steps and potential flaky tests need a concrete follow-up. Done should be a decided scope and documented or reproducible test findings.

Written by the indexing model from the issue text.