func test: fix min rbf feerate

Here, the min RBF feerate is 1 more than that of the transaction
to be replaced.

The feerate of the transaction to be replaced may vary slightly
depending on the signature size and is calculated during the test.
As such, the min feerate should be set according to this
calculated value.

tests/test_rpc.py

@@ -1145,7 +1145,8 @@ def test_rbfpsbt_bump_fee(lianad, bitcoind):
     # feerate to set the min feerate, instead of 1 sat/vb of first
     # transaction:
     with pytest.raises(
-        RpcError, match=f"Feerate {int(rbf_1_feerate)} too low for minimum feerate 10."
+        RpcError,
+        match=f"Feerate {int(rbf_1_feerate)} too low for minimum feerate {int(rbf_1_feerate) + 1}.",
     ):
         lianad.rpc.rbfpsbt(first_txid, False, int(rbf_1_feerate))
     # Using 1 more for feerate works.