Simplify the abc_mining_basic.py test

Summary: The test uses getblocktemplate which checks that the node is connected to at least one peer. Rather than spinning a node, just use the test framework to work around this check.

Test Plan:

./test/functional/test_runner.py abc_mining_basic

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11072