Function request() in phpbb_functional_test_case.php merges root url with path but does not remove single and double dots from path. As a result when both root_url and path are set it results in URLs like this:
Symphony's HTTP client requests it like that without removing ./ from URLs and in result $_SERVER variable is different, confusing Request class... or something like that. I'm not sure which one of those components is causing error.
Solution is to fix URLs before requesting it in test suite, avoiding that bug.