doc: update test shell binary names

[zhanzhao-h]

Description

Updates the TestShell documentation to use Bitgesell binary names consistently.

The document already refers to BGLd/BGL-cli in most places; this fixes the remaining bitcoind and bitcoin-cli references.

Notes

Docs-only change.

BTC/BGL PR reward address

To be provided if applicable.

Validation:

• git diff --check origin/master..doc/fix-test-shell-binary-names
• rg -n "bitcoind|bitcoin-cli" test/functional/test-shell.md

Refs #32

[MyTH-zyxeon]

Review-assist note for #32 / #81 / #340:

This is a narrow docs-only cleanup, but it targets a real contributor-facing mismatch: test/functional/test-shell.md should show the Bitgesell binary names that a BGL contributor actually has in this tree.

Acceptance checks I would use before merge/payout:

• Confirm the two changed examples now use BGLd / BGL-cli consistently with the rest of the TestShell document.
• Re-run the author's grep check and verify any remaining bitcoind / bitcoin-cli mentions in this file are intentionally absent or intentionally historical.
• Treat this as low-risk documentation polish under the BGL PR bounty hunt #32/BGL (Bitgesell) Bounty/Improvement Program ($50000 budget) #81 improvement lane; it should not affect consensus, wallet behavior, RPC behavior, or build outputs.

I did not build, start nodes, use wallet keys, or perform any live-chain action. This is just a maintainer-facing review checklist for a small documentation consistency PR.