tests: fix invalid key image test

It was using an invalid key image which wasn't in the key image
domain, so the transaction was rejected earlier than the test
was expecting

tests/core_tests/tx_validation.cpp

@@ -163,6 +163,15 @@ namespace
     throw std::runtime_error("invalid public key wasn't found");
     return crypto::public_key();
   }
+
+  crypto::key_image generate_invalid_key_image()
+  {
+    crypto::key_image key_image;
+    // a random key image plucked from the blockchain
+    if (!epee::string_tools::hex_to_pod("6b9f5d1be7c950dc6e4e258c6ef75509412ba9ecaaf90e6886140151d1365b5e", key_image))
+      throw std::runtime_error("invalid key image wasn't found");
+    return key_image;
+  }
 }
 
 //----------------------------------------------------------------------------------------------------------------------
@@ -541,8 +550,7 @@ bool gen_tx_key_image_is_invalid::generate(std::vector<test_event_entry>& events
   builder.step2_fill_inputs(miner_account.get_keys(), sources);
 
   txin_to_key& in_to_key = boost::get<txin_to_key>(builder.m_tx.vin.front());
-  crypto::public_key pub = generate_invalid_pub_key();
-  memcpy(&in_to_key.k_image, &pub, sizeof(crypto::ec_point));
+  in_to_key.k_image = generate_invalid_key_image();
 
   builder.step3_fill_outputs(destinations);
   builder.step4_calc_hash();