exp queue: Add two command queue start and stop (#7589)

fix: #7589

1. Add two sub-command `dvc queue start` and `dvc queue stop`
2. Add a unit test to test them

tests/unit/command/test_queue.py

@@ -1,6 +1,8 @@
 from dvc.cli import parse_args
 from dvc.commands.queue.kill import CmdQueueKill
 from dvc.commands.queue.remove import CmdQueueRemove
+from dvc.commands.queue.start import CmdQueueStart
+from dvc.commands.queue.stop import CmdQueueStop
 
 
 def test_experiments_remove(dvc, scm, mocker):
@@ -61,3 +63,35 @@ def test_experiments_kill(dvc, scm, mocker):
 
     assert cmd.run() == 0
     m.assert_called_once_with(revs=["exp1", "exp2"])
+
+
+def test_experiments_start(dvc, scm, mocker):
+    cli_args = parse_args(["queue", "start", "-j", "3"])
+    assert cli_args.func == CmdQueueStart
+
+    cmd = cli_args.func(cli_args)
+    m = mocker.patch(
+        "dvc.repo.experiments.queue.local.LocalCeleryQueue.spawn_worker",
+    )
+
+    assert cmd.run() == 0
+    assert m.call_count == 3
+
+
+def test_experiments_stop(dvc, scm, mocker):
+    cli_args = parse_args(
+        [
+            "queue",
+            "stop",
+            "--kill",
+        ]
+    )
+    assert cli_args.func == CmdQueueStop
+
+    cmd = cli_args.func(cli_args)
+    m = mocker.patch(
+        "dvc.repo.experiments.queue.local.LocalCeleryQueue.shutdown",
+    )
+
+    assert cmd.run() == 0
+    m.assert_called_once_with(kill=True)