[tests] Move stop_node and start_node methods to BitcoinTestFramework

Summary:
This commit moves functions start_node, start_nodes, stop_node and
stop_nodes functions into the BitcoinTestFramework class. It also moves
the bitcoind_processes dict and coverage variables into BitcoinTestFramework.

Backport core's PR10556

Depends on D990, D1000 and D992

Test Plan:

make check
./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1001