daemon-test-suite: parallelize tox env with -n 4 --dist loadgroup (#23512)

## Cause

The `daemon-test-suite` `py312` tox env runs ~527s on CI for 38 tests, all serial. `pytest-xdist` 3.6.1 is loaded transitively via `dagster[test]`, but the tox `commands = pytest -s -vv {posargs}` line has no `-n` flag, so xdist never gets used.

The 3 tests in `monitoring_daemon_tests/test_monitoring.py` share a session-scoped `hostnames` fixture wrapping `docker_compose_cm` — the same anti-pattern that broke PR #23506 (4 xdist workers each tried to bring up the same compose stack; 3 failed). This needs handling specifically; the remaining 35 tests don't share a docker compose stack and parallelize freely.

## Fix

- Change tox `commands =` to `pytest -s -vv -n 4 --dist loadgroup {posargs}`.
- Mark the 3 monitoring tests with `@pytest.mark.xdist_group(name="monitoring_compose")` so loadgroup co-locates them on a single worker (one compose stack per group, never racing).

`--dist loadgroup` (rather than `worksteal`) is the right scheduler when some tests *must* go to the same worker; pytest-xdist guarantees same-group tests share a worker. Worker count is fixed at 4 because `os.cpu_count()` inside K8s pod runners returns 1, defeating `-n auto`.

## Before / after

CI baseline: ~527s pytest serial (38 tests).

Estimated CI: ~3 minutes saved per run. The 3 monitoring tests stay on one worker (their docker compose runtime is the floor); the remaining 35 spread across the other three workers, so wall-clock goes from `serial(38)` to roughly `max(serial(3 monitoring), serial(35) / 3)`.

**Observed (job wall-clock, including tox setup):**

| Build | Branch | daemon-test-suite-3-12 |
|---|---|---|
| [155030](https://buildkite.com/dagster/internal/builds/155030) | master | 417s (6m 57s) |
| [155046](https://buildkite.com/dagster/internal/builds/155046) | this PR | **237s (3m 57s)** |

**−180s, 1.76× speedup.** Lands at the optimistic end of the predicted floor (`max(monitoring_compose ≈ 240s, test_memory ≈ 150s)`) — the `monitoring_compose` group is now the bottleneck as designed.

Local validation skipped — daemon-test-suite needs docker compose plus many dagster packages installed, so CI is the first real signal. Verified the Python `pytest.mark.xdist_group` decoration parses, and `tox.ini` parses as INI.

## Same assertions

No tests added, removed, skipped, marked `xfail`, or modified. The decorator only affects worker assignment; xdist scheduling is the only behavior change.

## Notes

`pytest-xdist` is already in the resolved deps via `dagster[test]` — no lockfile change required.

This PR explicitly avoids the trap that bit PR #23506: marking the docker-compose-bound tests with an `xdist_group` is the simple alternative to making the underlying fixture xdist-safe. PR #23507 takes the deeper-fix approach for the dagster-cloud-e2e suite where many more tests share the same compose stack.

Internal-RevId: 9974bcec3953a3c567ff2c23db405100ea16e3ec

Author: smackesey

integration_tests/test_suites/daemon-test-suite/monitoring_daemon_tests/test_monitoring.py

@@ -95,6 +95,7 @@ def log_run_events(instance, run_id):
             print(str(log) + "\n")  # noqa: T201
 
 
+@pytest.mark.xdist_group(name="monitoring_compose")
 def test_monitoring():
     # with setup_instance() as instance:
     with instance_for_test(
@@ -116,6 +117,7 @@ def test_monitoring():
             assert all_daemons_healthy(instance)
 
 
+@pytest.mark.xdist_group(name="monitoring_compose")
 def test_docker_monitoring(docker_postgres_instance, aws_env):
     docker_image = get_test_project_docker_image()
 
@@ -194,6 +196,7 @@ def test_docker_monitoring(docker_postgres_instance, aws_env):
                     assert instance.get_run_by_id(run.run_id).status == DagsterRunStatus.SUCCESS
 
 
+@pytest.mark.xdist_group(name="monitoring_compose")
 def test_docker_monitoring_run_out_of_attempts(docker_postgres_instance, aws_env):
     docker_image = get_test_project_docker_image()
 

integration_tests/test_suites/daemon-test-suite/tox.ini

@@ -28,4 +28,4 @@ allowlist_externals =
   uv
 commands =
   !windows: /bin/bash -c '! uv pip list --exclude-editable | grep -e dagster'
-  pytest  -s -vv  {posargs}
+  pytest  -s -vv -n 4 --dist loadgroup  {posargs}