Improve test coverage of the getblocktemplate longpoll feature with avalanche

Summary: These test cases check that avalanche plays nicely with longpoll, and ensure we don't introduce regressions.

Test Plan:

./test/functional/test_runner.py mining_getblocktemplate_longpoll

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17113