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/checksrc.yml

@@ -62,7 +62,7 @@ jobs:
             codespell python3-pip python3-networkx python3-pydot python3-yaml \
             python3-toml python3-markupsafe python3-jinja2 python3-tabulate \
             python3-typing-extensions python3-libcst python3-impacket \
-            python3-websockets python3-pytest
+            python3-websockets python3-pytest python3-filelock python3-pytest-xdist
           python3 -m pip install --break-system-packages cmakelint==1.4.3 pytype==2024.10.11 ruff==0.11.9
 
       - name: spellcheck