test: destroy_vm on a stopped VM no longer spins for a minute and gigabytes

test_destroy_vm_already_stopped left the stop task a bare MagicMock, so
_wait_for_task never saw "stopped" and looped for the full 60 s timeout
with time.sleep patched out. Every call landed in mock_calls: the test
took 60 s and up to 6 GB, and the new CI's 3.12 job was killed for it
(exit 137).

The stop call now raises, as Proxmox does for a VM that is not running --
which is the case the test is named for. The suite drops from 63 s to 4 s.

tests/test_vm_provision_mixin.py

@@ -459,7 +459,13 @@ def test_destroy_vm_success():
 
 
 def test_destroy_vm_already_stopped():
-    """destroy_vm succeeds even if VM already stopped."""
+    """destroy_vm succeeds even if VM already stopped.
+
+    Proxmox refuses to stop a VM that is not running, so the stop call raises.
+    (A bare MagicMock as the stop task never reports "stopped": _wait_for_task
+    then spun for the full timeout with sleep patched out, and every recorded
+    mock call made the test take 60 s and several GB, enough for CI to kill it.)
+    """
     mixin = ProxmoxVMProvisionMixin()
 
     mock_node = MagicMock()
@@ -468,6 +474,7 @@ def test_destroy_vm_already_stopped():
     # Mock VM operations
     mock_vm = MagicMock()
     mock_node.qemu.return_value = mock_vm
+    mock_vm.status.stop.post.side_effect = Exception("VM 101 not running")
     mock_vm.config.get.return_value = {}
     mock_vm.delete.return_value = None