Unify end-to-end tests under pytest

[bneradt]

This is a draft for the all-in end-to-end test migration discussed on the
original pytest replay PR. It supersedes #13035 without depending on changes to
the AuTest project.

Summary

• make pytest own the complete ATS end-to-end inventory
• collect all 96 tracked Proxy Verifier replays directly from .test.yaml
• collect each of the 505 bespoke .test.py definitions as one pytest item,
  using the pinned AuTest package only as a compatibility backend
• replace the separate parallel runner with pytest-xdist scheduling, stable CI
  sharding, isolated sandboxes/ports, and shared/exclusive serial-test locking
• rename the public interface to e2etest: e2etest.sh, CMake options and
  targets, presets, documentation, environment variables, and the replay
  e2etest: metadata key

Each replay file remains an independent item. A feature with several scenarios
should therefore keep separate scenario.test.yaml files unless the sessions
intentionally share ATS state.

Developer workflow

Run one test from the source tree:

./tests/e2etest.sh -f cache-control-basic
./tests/e2etest.sh -f cache-auth

The same -f workflow selects native YAML or compatibility Python items and
supports globs and multiple names. -k, -m, -j, --list, and other pytest
selection options remain available. A configured tree generates
<build>/tests/e2etest.sh for the fastest warmed-up single-test path.

Container behavior

The source entry point defaults to
ci.trafficserver.apache.org/ats/fedora:44, performs an incremental dedicated
build/install, and runs the selected tests there. It launches Docker with the
CI-relevant --init, host networking, and SYS_PTRACE capability, but does not
use --privileged or mount the Docker socket.

Automatic direct execution requires both a container marker and exact Fedora
44 identification, so the existing Jenkins container does not attempt Docker
in Docker. --run-in-docker and --no-run-in-docker override the decision.
Container-local short /tmp sandboxes avoid Unix-socket path and bind-mount
problems while regular diagnostics are copied back to the build tree.

CI compatibility

Temporary deprecated aliases retain ci-fedora-autest, autest.sh, the old
CMake options/targets, and the Jenkins parallel_mode feature probe. This lets
the current external pipeline execute the unified pytest inventory immediately;
the pipeline can rename its stage, preset, and command in a follow-up. Existing
SHARD/SHARDCNT values now shard all pytest items rather than only .test.py
files.

Validation

• Fedora 44 Docker build, install, and 627-item local collection (the worktree
  contains one unrelated untracked replay in addition to this PR's 626 items)
• all replay items passed or reached their expected capability skips
• mixed native/compatibility selections passed under xdist
• representative DNS, config-reload, next-hop, and plugin compatibility tests
  passed concurrently
• current Jenkins-style 3/12 compatibility shard collected 52 items
• 25 framework tests and Python static checks passed
• new and deprecated Fedora CMake presets configured successfully
• repository formatting and whitespace hooks passed

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