Restrict setmocktime to non-negative integers

Summary:
The assumption that now > 0 is applied throughout util/time.cpp,
so there's no reason to expect setting mock time to a negative int to be valid.

Test Plan: ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6022