Rename the bitcoind-era public test-framework API to Z3 names

[dannywillems]

Feel free to close if you disagree with the renaming - don't lose much time reading if it is contraversary, I don't have a strong opinion. This is an automated patch generated by Claude in the background while onboarding on the code.

---

Closes #110. Builds on #108 (stacked: base is chore/rename-node-helpers;
retarget to main once #108 merges).

The test framework descends from Bitcoin Core via zcashd and kept the
inherited naming even though the node under test is zebrad. This renames
the widely-used public API for clarity:

• BitcoinTestFramework -> ZcashTestFramework
• bitcoind_processes -> node_processes
• wait_bitcoinds -> wait_nodes

Plus node-related comments, docstrings, and a loop variable in the
framework (e.g. "Start a bitcoind" -> "Start a node").

Mechanical, repo-wide rename (~129 files). Left unchanged on purpose:
copyright headers ("The Bitcoin Core developers"), references to external
projects (python-bitcoinrpc, bitcoinj, bitcoinlib), the ZCASHD_BINARY
constant used by not-yet-migrated EXTENDED tests, and the already-accurate
node-generic names (start_node/start_nodes, connect_nodes).

Verified locally: all files parse and getmininginfo.py / nuparams.py
still pass.

[oxarbitrage]

Reviewed this mechanically — compiled all 129 changed files (all parse) and swept the whole repo for each renamed symbol. Clean rename, with one miss:

Breaks qa/zcash/create_wallet_200k_utxos.py. The sweep covered qa/rpc-tests/ but not this helper under qa/zcash/, which subclasses the framework with the old name (now removed — no back-compat alias):

• L16 from test_framework.test_framework import BitcoinTestFramework → ImportError
• L27 class LargeWalletTest(BitcoinTestFramework):

Two-line fix (BitcoinTestFramework → ZcashTestFramework). It's a benchmark helper, not a CI rpc-test, so CI won't flag it.

Nit: qa/pull-tester/rpc-tests.py:15 docstring still references BitcoinTestFramework.main (pre-existing, and the path there is stale too) — worth a touch-up while renaming.

Everything else checks out — bitcoind_processes, wait_bitcoinds, wait_for_bitcoind_start, zcashd_binary() fully renamed (0 stragglers); ZCASHD_BINARY constant, start_node/start_nodes, and copyright headers correctly left alone.

Suggestion: grep the whole repo (not just qa/rpc-tests/) for the old names so any framework users outside that dir get caught.

On timing — since this rewrites the same core files as the in-flight #56 and #104, it might be cleanest to land those first and rebase this on top (the rename is trivially regenerable), so the functional PRs don't eat the conflicts. No strong opinion, just a sequencing thought.