fix: a pr2444 introduced valgrind complaint

This will add the testnet config copyNpaste to test_utils.c,
so that the test stups can set these.

Alternatively, to reduce code duplication, we can move the
testnet_config and mainnet_config from options.c to options.h.

wallet/test/test_utils.h

@@ -1,6 +1,8 @@
 #ifndef LIGHTNING_WALLET_TEST_TEST_UTILS_H
 #define LIGHTNING_WALLET_TEST_TEST_UTILS_H
 
+#include "lightningd/lightningd.h"
+
 /* Definitions "inspired" by libsecp256k1 */
 #define TEST_FAILURE(msg) do { \
     fprintf(stderr, "%s:%d: %s\n", __FILE__, __LINE__, msg); \
@@ -21,4 +23,6 @@
 
 #define CHECK(cond) CHECK_MSG(cond,"test condition failed");
 
+const struct config test_config;
+
 #endif /* LIGHTNING_WALLET_TEST_TEST_UTILS_H */