test: Actually set sequence in MiniWallet create_self_transfer_multi #26278

Can be tested with:

diff --git a/test/functional/test_framework/wallet.py b/test/functional/test_framework/wallet.py
index de8eedc10e..25b630bb00 100644
--- a/test/functional/test_framework/wallet.py
+++ b/test/functional/test_framework/wallet.py
@@ -284,6 +284,7 @@ class MiniWallet:
             self._bulk_tx(tx, target_weight)
 
         txid = tx.rehash()
+        assert [i.nSequence for i in tx.vin]==sequence
         return {
             "new_utxos": [self._create_utxo(
                 txid=txid,

Not such a big of an issue (as I believe other tests would break if that was the case) but this test scenario is not tested since #25522 where this was introduced.

I believe that the current logic of how create_self_transfer_multi uses create_self_transfer in order to create a simple transaction template to then adjust (copy and adapt) based on its arguments can more easily lead to this kind of issues and is more of a maintenance burden.

Looking into this, I've came up with a change in logic, where create_self_transfer uses create_self_transfer_multi. I think this simplifies the code and (if I am not missing something) could be a replacement to this PR. You can see this change in https://github.com/kouloumos/bitcoin/commit/0b78110f73965fd827748107e0f3497d3352be71 (modify-create_self_transfer branch).

Sounds good