Merge bitcoindevkit/bdk#2394: test(chain): reuse indexer helper in apply_changeset...

b7931c88fe70a1a4633873b5cf6fab805ff4a62a test(chain): reuse indexer helper in apply_changeset clamp test (busayo-OD)

Pull request description:

  ### Description

  Reuse indexer_with_last_revealed_at in apply_changeset_clamps_out_of_range_index_and_derives_correct_spk to simplify the test setup. The helper is also moved above its first use for readability.

  Follow-up to #2265.

  ### Checklists

  #### All Submissions:

  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)

ACKs for top commit:
  evanlinjin:
    ACK b7931c88fe70a1a4633873b5cf6fab805ff4a62a

Tree-SHA512: 9960ae6ee4813617e107cf050786b284dc75784a5d2f54008a4fa755972e44805518d588eb09b1defdf88fb050072ab56925b6d11500e9fe7dc43549a02f7931