test: add xfail test for gevent + asyncio event loop in greenlet bug

[freider]

Summary

Adds a new xfail test that captures a known bug: when gevent (monkey.patch_all()) is active and an asyncio event loop is running inside one greenlet, calling synchronicity blocking functions from another greenlet fails with:

RuntimeError: asyncio.run() cannot be called from a running event loop

Root Cause

Under gevent, asyncio._get_running_loop() is patched to be visible globally (not just to the thread running the loop). So when any greenlet calls asyncio.run() internally (as synchronicity does), it sees the event loop running in another greenlet and raises RuntimeError.

Changes

• test/support/_gevent_asyncio_in_greenlet.py: New script that reproduces the bug — spawns a greenlet with a running asyncio loop and calls a synchronicity blocking function from another greenlet simultaneously.
• test/gevent_test.py: New test test_gevent_asyncio_in_greenlet marked @pytest.mark.xfail(strict=True) with the same skip conditions as test_gevent.

Status

Intentionally xfail — documents the bug. Fix to follow in a separate PR.

_{Sent by @freider from gevent/asyncio RuntimeError investigation.}