pytest-xdist: pytest in parallel

Require now pytest-xdist from tests/http/requirements.txt and
run pytest in 'auto' parallel mode (counts cpu cores).

For CI runs, set the worker count to 4, overriding the
core count of 2 exposed in the images.

- use Filelock to generate allocated ports at start for all
  workers and have subsequent workers just read the file and
  take the ports for their slot
- make httpd config clearing a function fixture so every test
  starts with a clean httpd config
- have fixture `configures_httpd` as parameter of test cases
  that configure httpd anyway, saving one reload
- add pytest-xdist and filelock to required pyhton modules
- add installs to ruff CI
- give live checks waiting for a server to start up longer time
- add fixtures to tests that rely on a server
- do not stop servers unnecessarily. failures may not start them
  properly again, leading to unexpected fails in whatever follows
- add a https: port to httpd that is *not* back by QUIC to allow
  failover tests without stopping the QUIC server

Closes #17295

.github/workflows/macos.yml

@@ -337,6 +337,7 @@ jobs:
         env:
           CURL_CI: github
           PYTEST_ADDOPTS: '--color=yes'
+          PYTEST_XDIST_AUTO_NUM_WORKERS: 4
         run: |
           source $HOME/venv/bin/activate
           if [ -n '${{ matrix.build.generate }}' ]; then