Add BitcoinApplication & RPCConsole tests

Summary:

Add test coverage for Qt initialization code & basic RPC console
functionality.

Completes backport of core PR11625 (commit 7e4bd19):
https://github.com/bitcoin/bitcoin/pull/11625/commits/7e4bd19785ff9120b7242ff9f15231868aaf7db4

Test Plan:

make check
ninja check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4502