Fix a bug where running test_runner.py --usecli would fail when built without bitcoin-cli

Summary:
Introduced in D2696. The existence check for bitcoin-cli did not account for --usecli.

Depends on D5781

Test Plan:

cmake -GNinja -DBUILD_BITCOIN_CLI=OFF ..
test_runner.py

This fails pre-patch and passes post-patch.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5788