test: Mark test_runs_concurrently with run_alone

Timing-sensitive test fails when running in parallel due to CPU
contention (0.312s > 0.3s threshold).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: vdusek

tests/unit/_autoscaling/test_autoscaled_pool.py

@@ -34,6 +34,7 @@ def future(value: T, /) -> Awaitable[T]:
     return f
 
 
+@pytest.mark.run_alone
 async def test_runs_concurrently(system_status: SystemStatus | Mock) -> None:
     done_count = 0