Add test coverage for getblocktemplate's mintime

Summary:
mintime is not tested anywhere. Rather than introduce a new merge conflict
in mining_basic.py we can test it here.

Part of T1014

Depends on D7230

Test Plan: test_runner.py abc_mining_basic

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7233