Make the longpoll much faster on regtest

Summary: This dramatically reduces the functional test duration, which will no longer be part of the extended test suite.

Test Plan:

./test/functional/test_runner.py mining_getblocktemplate_longpoll
ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16542