Add configuration/argument testing

Summary:
Adds a new functional test for testing various command line and
configuration file argument interactions, that aren't specific enough to
other functionality to be placed in other tests.

Currently this tests the error messages for non-existent datadir

Backport of core PR11883

Test Plan:

./test/functional/test_runner.py conf_args

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2325