tests: replace mockmakedate function in test-amend.t

Authored by taapas1128.

Description

tests: replace mockmakedate function in test-amend.t

This is a follow up patch for D5554. This replaces mockmakedate function
by tests/mockmakedate.py.

Differential Revision: https://phab.mercurial-scm.org/D5577

Details