test: cover failed CDBIterator::GetKey() deserialization

The upcoming change will replace the temporary owning `DataStream` inside `CDBIterator::GetKey()` with a borrowed reader over the current LevelDB key bytes.
The copied `DataStream` currently insulates the iterator entry from a failed decode, so the optimization is only safe if a deserialization failure still returns `false` and leaves the same key/value readable afterward.

Extend `dbwrapper_iterator` to read a one-byte key as a `uint16_t`.
The read must fail, return `false`, and still allow the same key and value to be read afterward.
This would fail if `GetKey()` stopped swallowing deserialization exceptions, or if a failed decode started consuming shared iterator state instead of only temporary reader state.

Drop the dead `const_cast` in the test while here, since `dbw` is already non-const.

Locking down that contract first makes the following `SpanReader` switch a behavior-preserving optimization.

src/test/dbwrapper_tests.cpp

@@ -201,11 +201,15 @@ BOOST_AUTO_TEST_CASE(dbwrapper_iterator)
         uint256 in2 = m_rng.rand256();
         dbw.Write(key2, in2);
 
-        std::unique_ptr<CDBIterator> it(const_cast<CDBWrapper&>(dbw).NewIterator());
+        std::unique_ptr<CDBIterator> it(dbw.NewIterator());
 
         // Be sure to seek past the obfuscation key (if it exists)
         it->Seek(key);
 
+        // A failed key decode must not consume the current iterator entry.
+        uint16_t key_too_large{0};
+        BOOST_CHECK(!it->GetKey(key_too_large));
+
         uint8_t key_res;
         uint256 val_res;