Fix flaky isolation_metadata_sync_deadlock (#7312)

Sometimes isolation_metadata_sync_deadlock fails in CI like this:

```diff
diff -dU10 -w /__w/citus/citus/src/test/regress/expected/isolation_metadata_sync_deadlock.out /__w/citus/citus/src/test/regress/results/isolation_metadata_sync_deadlock.out
--- /__w/citus/citus/src/test/regress/expected/isolation_metadata_sync_deadlock.out.modified	2023-11-01 16:03:15.090199229 +0000
+++ /__w/citus/citus/src/test/regress/results/isolation_metadata_sync_deadlock.out.modified	2023-11-01 16:03:15.098199312 +0000
@@ -110,10 +110,14 @@
 t
 (1 row)

 step s2-stop-connection:
  SELECT stop_session_level_connection_to_node();

 stop_session_level_connection_to_node
 -------------------------------------

 (1 row)
+
+teardown failed: ERROR:  localhost:57638 is a metadata node, but is out of sync
+HINT:  If the node is up, wait until metadata gets synced to it and try again.
+CONTEXT:  SQL statement "SELECT master_remove_distributed_table_metadata_from_workers(v_obj.objid, v_obj.schema_name, v_obj.object_name)"
```

Source:
https://github.com/citusdata/citus/actions/runs/6721938040/attempts/1#summary-18268946448

To fix this we now wait for the metadata to be fully synced to all
nodes at the start of the teardown steps.

(cherry picked from commit a6e8688)

src/test/regress/spec/isolation_metadata_sync_deadlock.spec

@@ -22,6 +22,7 @@ setup
 
 teardown
 {
+  SELECT wait_until_metadata_sync();
   DROP FUNCTION trigger_metadata_sync();
   DROP TABLE deadlock_detection_test;
   DROP TABLE t2;