Add a test for ABC-specific getblocktemplate behavior

Summary:
First step in T1014

As we modify behavior in getblocktemplate, we really need better test coverage.
D7201 (f8360b5e981) is one such change. This patch tests that behavior specifically
but this test can be expanded to cover other changes that we may have made or will
make in the future.

Test Plan:
Before D7201, this test should fail

git checkout f8360b5e981^
git cherry-pick this-patch
test_runner.py abc_mining_basic

But the test passes today:

git checkout this-patch
test_runner.py abc_mining_basic
test_runner.py --with-axionactivation abc_mining_basic

Reviewers: #bitcoin_abc, Fabien, deadalnix

Reviewed By: #bitcoin_abc, Fabien, deadalnix

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7230