tests: stop ps immediately in test_ps_unavailable_after_delete (#12728)

## Problem
test_ps_unavailable_after_delete is flaky. All test failures I've looked
at are because of ERROR log messages in pageserver, which happen because
storage controller tries runs a reconciliations during the graceful
shutdown of the pageserver.

I wasn't able to reproduce it locally, but I think stopping PS
immediately instead of gracefully should help. If not, we might just
silence those errors.

- Closes: https://databricks.atlassian.net/browse/LKB-745

test_runner/regress/test_storage_controller.py

@@ -3309,6 +3309,7 @@ def test_ps_unavailable_after_delete(
         ps.allowed_errors.append(".*request was dropped before completing.*")
         env.storage_controller.node_delete(ps.id, force=True)
         wait_until(lambda: assert_nodes_count(2))
+        env.storage_controller.reconcile_until_idle()
     elif deletion_api == DeletionAPIKind.OLD:
         env.storage_controller.node_delete_old(ps.id)
         assert_nodes_count(2)