Apache httpd existed many years before junit was invented in 1997. Long before TDD became a thing and our rigorous modern understanding set in. For a second, I even thought the Apache Foundation (founded because of httpd) later hosted junit, but I was wrong, is the Eclipse Foundation.
Though I agree, that although not a technical justification, an explanation as to why there are no tests is because Apache HTTP is from the 90's. Not writing unit tests was par for course back then. Most FLOSS code bases in the 90s didn't have unit tests, let a alone a CI to run the test suite for each change. Adding tests later is hard. Though there are some tests under the test folder.
Some people were writing their own ad-hoc scaffolds before that but it wasn't a widespread practice. Testing meant manual testers clicking on things in the UI, and sophisticated testing was if you had checklists of things to test manually.