Fix incorrect mocktime set in miner fund test
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

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