[backport#15896] QA: feature_filelock, interface_bitcoin_cli: Use PACKAGE_NAME in messages rather than hardcoding Bitcoin Core

Summary:
QA: feature_filelock, interface_bitcoin_cli: Use PACKAGE_NAME in messages rather than hardcoding Bitcoin Core (Luke Dashjr)

https://github.com/bitcoin/bitcoin/pull/15896/commits/fcc443b6367b44d5fdc04db36ec4a286d5b59162

Backport of Core PR15896

Test Plan:

ninja clean && ninja
./test_runner.py feature_filelock interface_bitcoin_cli

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7107