tests: add curated bitcoin core test set

After analyzing countless Bitcoin Core tests from test/functional
folder, invariants were picked for corepc such that if a particular
assertion failed, that means corepc has a bug. The tests are
complementary and do not repeat or collide with existing corepc
integration tests.

Identical test copies from core proved unproductive, extense and
tested much more than necessary for corepc. All existing tests are
derivations centered around checking corepc honors a particular
invariant. Optional fields were specially targeted to prevent absence
failures that into_model() conversions failed to catch. Some setups
exercise previously untested fields.

Author: satsfy

integration_test/README.md

@@ -33,3 +33,8 @@ alias test28='BITCOIND_EXE=/opt/bitcoin-28.2/bin/bitcoind cargo test --features=
 alias test29='BITCOIND_EXE=/opt/bitcoin-29.0/bin/bitcoind cargo test --features=29_0'
 alias test30='BITCOIND_EXE=/opt/bitcoin-30.2/bin/bitcoind cargo test --features=30_2'
 ```
+
+## Bitcoin Core Tests
+
+Tests derived from Bitcoin Core's `test/functional/` folder live in
+`tests/` alongside the modelled tests, with a `_core` filename suffix.