test: use BGL unit test binary names

[JONASXZB]

Summary

This is a small test/documentation consistency cleanup for the Bitgesell tree:

• Replace leftover test_bitcoin examples with the actual Bitgesell unit test binary name, test_BGL.
• Fix the malformed test_ --run_test=... example in src/test/README.md.
• Align the documented unit-test LCOV output path with the existing Makefile.am target, test_BGL.coverage.
• Update a Makefile comment and unit-test helper comments to match the current Bitgesell binary names.

This is intentionally scoped to docs/comments only; it does not change runtime behavior.

Validation

grep -RIn "test_bitcoin\|test_common_Bitcoin Core\|test_ --run_test" src/test src/Makefile.test.include doc/developer-notes.md 2>/dev/null || true
python3 - <<'PY'
from pathlib import Path
makefile = Path('Makefile.am').read_text()
docs = Path('doc/developer-notes.md').read_text()
assert 'test_BGL.coverage/.dirstamp' in makefile
assert './test_BGL.coverage/index.html' in docs
print('coverage doc matches Makefile target')
PY
git diff --check

Result: no stale test_bitcoin / test_common_Bitcoin Core / malformed test_ --run_test references remain in the touched test-doc scope; coverage doc matches the Makefile target; diff check passes.

Bounty note

If this small cleanup qualifies under the Bitgesell PR/improvement bounty program (#39 / #81), public payout addresses are:

• USDT TRC20: TU3WsDnY9RbwGFEHx6WoF1C7J592SGrS8F
• USDT/ERC20-compatible EVM: 0xc580927355132643ad82b4c12ba94a7a93e63ba2

[MyTH-zyxeon]

Maintainers - this looks like the cleanest first landing candidate in the Bitgesell #81 improvement queue right now.

Why this seems low-risk and reviewer-friendly:

• Scope is narrow and explicit: docs/comments and unit-test command references are aligned to the actual test_BGL binary naming, without changing consensus, wallet behavior, or runtime logic.
• PR state is currently OPEN, mergeStateStatus=CLEAN, mergeable=MERGEABLE, and there are no visible review threads yet.
• Because the bounty program issue examples explicitly mention test-fix / cleanup work as acceptable, this kind of consistency fix looks like a reasonable small first merge if you want to clear one item quickly.

Suggested acceptance checks:

1. Confirm every renamed command/reference matches the binary names actually produced by the current test/build flow.
2. Confirm there are no stale references left in nearby docs/comments/scripts that still point to the old naming.
3. If you treat this as a paid BGL (Bitgesell) Bounty/Improvement Program ($50000 budget) #81 slice, confirm whether docs/test-command consistency work is eligible at the same tier as the example small test fixes, or at a reduced amount.

If you prefer a different first landing candidate under #81, calling that out here should make it easier to route future small PRs to the right repo without duplicating review effort.