test: add CONST_SCRIPTCODE failure-path vectors to script_tests.json #35835

Follow-up to #35664.

While going through which script error codes are covered by src/test/data/script_tests.json, I noticed that the two errors gated behind SCRIPT_VERIFY_CONST_SCRIPTCODE are never asserted anywhere in the test suite:

• SCRIPT_ERR_OP_CODESEPARATOR shows up only once, as a mempool reject string in test/functional/data/invalid_txs.py. Nothing asserts it at the script level.
• SCRIPT_ERR_SIG_FINDANDDELETE is not asserted by any test.

tx_invalid.json does contain a SCRIPT_VERIFY_CONST_SCRIPTCODE section, but those tests can only state that a transaction is invalid, not which error made it fail. script_tests.json is the harness that verifies exact error codes, and today it has no vector that uses the CONST_SCRIPTCODE flag.

This PR adds one section with six static vectors:

• OP_CODESEPARATOR in an executed pre-segwit script fails with its own error code.
• OP_CODESEPARATOR inside an unexecuted IF branch fails as well: the check in EvalScript happens before the fExec guard, so the opcode is rejected even though it never runs. This rule had no error-level coverage before.
• A control vector: the same unexecuted-branch script passes when the flag is off.
• A signature push that also appears in the scriptPubKey makes CHECKSIG fail with SIG_FINDANDDELETE, since FindAndDelete locates the signature inside the scriptCode.
• The same case for CHECKMULTISIG, which calls FindAndDelete for each signature in its own loop.
• A control vector: with the flag off, FindAndDelete silently removes the push and the script still evaluates.

Both checks trigger before any signature verification happens, so the vectors can rely on a dummy signature — the same approach used in #35664.

To confirm that the expected errors are the ones that really fire, I first added the vectors expecting OK, let the harness report the actual error for each one, and then set the expectations accordingly. Tested with:

cmake --build build --target test_bitcoin
build/bin/test_bitcoin --run_test=script_tests/script_json_test

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35835.

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