test(bigtable): fix accelerator system-test failures (script import, asyncio loop scope, fd-leak settle) - #21

Stacked on #19 (accel-18). Fixes three failures surfaced running the accelerator
system suite under the current CI environment (newer pytest / pytest-asyncio).

1. stress driver as a standalone script — stress.py used a top-level
   from . import _harness, which fails with ImportError: attempted relative import with no known parent package when run as python stress.py (its
   docstring markets hand-running). Guarded on __package__ so both
   python -m tests.system.data.accelerator.stress and python stress.py work.

2. pytest-asyncio loop scope — the system suites share session-scoped async
   client/table fixtures, which require the tests and those fixtures to run on
   one event loop. pytest-asyncio <=0.21 enforced this via the session-scoped
   event_loop override in tests/system/conftest.py; pytest-asyncio >=0.23
   ignores that override, so fixtures ended up on a different loop than the tests
   (got Future <...> attached to a different loop). Added pytest.ini pinning
   asyncio_default_fixture_loop_scope/asyncio_default_test_loop_scope to
   session, plus a root conftest.py that registers those ini options so older
   pytest-asyncio doesn't emit an unknown-option warning (fatal under -W error).
   No-op on 0.21.2 (keys ignored, override still drives it); full unit suite
   (5509) verified green under the config.

3. fd-leak settle — test_no_leaks_over_repeated_lifecycles failed on fd
   count (30->40) while daemon-subprocess and tempdir leak checks passed. That is
   the known async-grpc fd high-water (channel fds released lazily on teardown),
   not a daemon leak. assert_no_leaks now GCs and takes the minimum fd count
   over a short settle window: a transient teardown high-water drains and passes,
   while a genuine per-cycle leak stays high and still fails.

Note: the standard nox system session pins pytest-asyncio==0.21.2, so the real
environmental cause is a CI harness that installs it unpinned; these repo-side
fixes make the suite robust across both.