test: use BGL utility names in util tests

[Sikkra]

The util test data and runner still had a small group of upstream Bitcoin binary names after the Bitgesell rename. BGL-util-test.json mostly runs ./BGL-util and ./BGL-tx, but the replaceable-input test block still asked the runner to execute ./bitcoin-tx. The runner also only honored BITCOINUTIL / BITCOINTX environment overrides, so local override runs had to use the old upstream names.

This updates the remaining util test entries and environment override hooks to the Bitgesell binary names:

• replace remaining ./bitcoin-tx entries in test/util/data/BGL-util-test.json with ./BGL-tx
• honor BGLUTIL for ./BGL-util
• honor BGLTX for ./BGL-tx

Verification:

• python -m py_compile test\util\test_runner.py
• parsed test/util/data/BGL-util-test.json and confirmed all 103 entries now use only ./BGL-util or ./BGL-tx

Bounty reference: #39