make NodeBackup aware of inheritors of PostgresNode

Author: funbringer

Diff for: testgres/backup.py

@@ -143,8 +143,8 @@ def spawn_primary(self, name=None, destroy=True):
         base_dir = self._prepare_dir(destroy)
 
         # Build a new PostgresNode
-        from .node import PostgresNode
-        with clean_on_error(PostgresNode(name=name, base_dir=base_dir)) as node:
+        NodeClass = self.original_node.__class__
+        with clean_on_error(NodeClass(name=name, base_dir=base_dir)) as node:
 
             # New nodes should always remove dir tree
             node._should_rm_dirs = True