98a1846b00 tests: Support calling add_nodes more than once (Steven Roose)
Pull request description:
Ran into this while writing [a multi-chain test for Elements](https://github.com/ElementsProject/elements/pull/458) where I call this method more than once.
Tree-SHA512: f2d698fcb560552aa5d81a4c3fbf40b7269b228b34d85a118291649ef83f8c0a30cd82a28d418237b55893bcecd538046b704e64a4d8a41f2c0aef8033dc83e5