qa: Move common args to bitcoin.conf
Summary:
Beside removing duplicates of the same args in the code, this actually helps with debugging after a test failure. For example, bitcoin-qt has server turned off, so you'd have to turn it on every time, if you wanted to debug a temporary test datadir created by the test framework. Also, keypool would fill up if you forget to specify -keypool=1.
Backport of core PR12443
https://github.com/bitcoin/bitcoin/pull/12443/files
Test Plan:
./test/functional/test_runner.py --extended
Reviewers: #bitcoin_abc, deadalnix, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D3518