Use correct min/max htlc in test util for constructing blinded pay params.

Use correct min/max htlc in test util for constructing blinded pay params.

In testing, we use channel updates to construct blinded paths and the
{Forward,Receive}Tlvs encoded within. Given a blinded path from node A > B > C,
we currently use channel_update_A->B to construct the payment constraints for
A’s blinded payload.

This is incorrect for setting A's PaymentConstraints::htlc_minimum_msat,
because channel_update_A->B contains the minimum value that *B* will accept,
and we want the constraints to contain the min value that *A* will accept.

This never caused test failures before because min/max htlc values were always
identical in both channel directions.

Therefore, set A’s htlc min/max values to the min/max that A will accept.