tests: Add fuzzing harness for various hex related functions

src/test/fuzz/hex.cpp

@@ -0,0 +1,22 @@
+// Copyright (c) 2019 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <test/fuzz/fuzz.h>
+
+#include <util/strencodings.h>
+
+#include <cassert>
+#include <cstdint>
+#include <string>
+#include <vector>
+
+void test_one_input(const std::vector<uint8_t>& buffer)
+{
+    const std::string random_hex_string(buffer.begin(), buffer.end());
+    const std::vector<unsigned char> data = ParseHex(random_hex_string);
+    const std::string hex_data = HexStr(data);
+    if (IsHex(random_hex_string)) {
+        assert(ToLower(random_hex_string) == hex_data);
+    }
+}