extrakeys: Init empty experimental module

This is to prepare for xonly_pubkeys and keypairs.

src/modules/extrakeys/tests_impl.h

@@ -0,0 +1,16 @@
+/**********************************************************************
+ * Copyright (c) 2020 Jonas Nick                                      *
+ * Distributed under the MIT software license, see the accompanying   *
+ * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
+ **********************************************************************/
+
+#ifndef _SECP256K1_MODULE_EXTRAKEYS_TESTS_
+#define _SECP256K1_MODULE_EXTRAKEYS_TESTS_
+
+#include "secp256k1_extrakeys.h"
+
+void run_extrakeys_tests(void) {
+    /* TODO */
+}
+
+#endif