test, build: Separate read_json function into its own module

src/test/key_io_tests.cpp

@@ -8,6 +8,7 @@
 #include <key.h>
 #include <key_io.h>
 #include <script/script.h>
+#include <test/util/json.h>
 #include <test/util/setup_common.h>
 #include <util/strencodings.h>
 
@@ -15,8 +16,6 @@
 
 #include <univalue.h>
 
-UniValue read_json(const std::string& jsondata);
-
 BOOST_FIXTURE_TEST_SUITE(key_io_tests, BasicTestingSetup)
 
 // Goal: check that parsed keys match test payload