tests: use dynamic ports numbers in pytest suite

- necessary ports are bound at start of test suite and then
  given to server fixtures for use.
- this make parallel use of pytest (in separate directories),
  practically safe for use as OS tend to not reuse such port numbers
  for a while

Closes #10692

tests/http/test_05_errors.py

@@ -79,7 +79,7 @@ class TestErrors:
         curl = CurlClient(env=env)
         urln = f'https://{env.authority_for(env.domain1, proto)}' \
                f'/curltest/tweak?id=[0-{count - 1}]'\
-               '&chunks=3&chunk_size=16000&body_error=reset'
+               '&chunks=5&chunk_size=16000&body_error=reset'
         r = curl.http_download(urls=[urln], alpn_proto=proto, extra_args=[
             '--retry', '0', '--parallel',
         ])