change TestLockPointValidity to take a const reference

Summary:
The lockpoints are not changed in this function.
There is no reason to pass a pointer.

Note for review: this is backported out of order after core#23683

This concludes backport of core#23649
https://github.com/bitcoin/bitcoin/pull/23649/commits/c4efc4db5484910ac33ba41aa76f4e23639d6f33

Depends on D12448

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12453