Fix incorrect mocktime set in miner fund test

Summary:
node.setmocktime() is called but the node is restarted immediately after.
Move setting the mock time to after the node starts back up since it's restarted many
times in this test.

Test Plan: test_runner.py abc-miner-fund

Reviewers: #bitcoin_abc, nakihito, Fabien

Reviewed By: #bitcoin_abc, nakihito, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6090