test: normalize previous release binary names

[CDEN190]

Description

Normalize previous-release binaries extracted or built by test/get_previous_releases.py from upstream Bitcoin names to the Bitgesell names used by the functional test framework.

Without this, --previous-releases can populate bin/bitcoind and bin/bitcoin-cli, while the test framework looks for bin/BGLd and bin/BGL-cli for versioned nodes.

This adds a small helper plus unit coverage for the rename behavior.

Validation

• python test\util\get_previous_releases_test.py
• python -m py_compile test\get_previous_releases.py test\util\get_previous_releases_test.py
• python test\lint\lint-tests.py
• git diff --check

Refs #32

[MyTH-zyxeon]

Maintainer review-assist for the #32 Bitgesell PR bounty queue.

Current visible state:

• PR is OPEN / MERGEABLE / CLEAN at head fb085f92465887ba843d162ff7345507c8ecbc4f.
• Diff is focused: test/get_previous_releases.py and test/util/get_previous_releases_test.py, +73/-2, no production consensus/runtime path touched.
• No visible PR comments or status checks were present when checked.

Review focus:

• The helper maps prior-release upstream Bitcoin binary names (bitcoind, bitcoin-cli, bitcoin-tx, bitcoin-wallet) to the BGL names the functional test framework expects (BGLd, BGL-cli, BGL-tx, BGL-wallet). That directly addresses previous-release functional-test compatibility for extracted and locally-built release bins.
• The non-overwrite guard is important: if a BGL-named binary already exists, the helper leaves both files in place instead of replacing the BGL binary. The new unit test covers that behavior, so maintainers should confirm that keeping the old upstream-named file is intentional rather than preferring cleanup/removal.
• build_release() now moves both upstream and BGL names if present before normalizing. That looks safer across mixed release artifacts, but the acceptance check should include one real --previous-releases smoke path or at least the reported python test\util\get_previous_releases_test.py plus python -m py_compile ... and python test\lint\lint-tests.py.

Suggested next step: run the focused Python helper tests and one previous-release functional-test setup path, then classify this as a small #32 test/devtools unblock if the non-overwrite behavior matches maintainer expectations.