exp: add test_queue unit test helper for celery queue

tests/unit/repo/experiments/conftest.py

@@ -0,0 +1,29 @@
+from contextlib import contextmanager
+
+import pytest
+
+from dvc.repo.experiments.queue.local import LocalCeleryQueue
+
+
+@contextmanager
+def _thread_worker(app, **kwargs):
+    # Based on pytest-celery's celery_worker fixture but using thread pool
+    # instead of solo pool so that broadcast/control API is available
+    from celery.contrib.testing import worker
+
+    app.loader.import_task_module("celery.contrib.testing.tasks")
+    with worker.start_worker(app, pool="threads", **kwargs) as test_worker:
+        yield test_worker
+
+
+@pytest.fixture
+def test_queue(tmp_dir, dvc, scm, mocker) -> LocalCeleryQueue:
+    """Patches experiments celery queue for pytest testing.
+
+    Test queue worker runs for the duration of the test in separate thread(s).
+    """
+    celery_queue = dvc.experiments.celery_queue
+    mocker.patch.object(celery_queue, "spawn_worker")
+    with _thread_worker(celery_queue.celery, concurrency=1) as worker:
+        mocker.patch.object(celery_queue, "worker", return_value=worker)
+        yield celery_queue