[util] Change GetMockTime to return chrono type instead of int

Summary:
This is a backport of core#21121 [3/4]
https://github.com/bitcoin/bitcoin/pull/21121/commits/df6a5fc1dff3b1b7c2f2b67aad1ff17cac99f7b6

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12108