[fuzz] Add serialization to deserialization test

Before commit:
306853	REDUCE cov: 798 ft: 5820 corp: 150/375Kb lim: 68333 exec/s: 1382 rss: 461Mb L: 254/63171 MS: 1 EraseBytes-
1453105	REDUCE cov: 798 ft: 5820 corp: 150/369Kb lim: 79613 exec/s: 1467 rss: 461Mb L: 6027/60873 MS: 1 EraseBytes-

After commit:
303389	NEW    cov: 1202 ft: 8382 corp: 157/382Kb lim: 68189 exec/s: 1451 rss: 447Mb L: 1386/65459 MS: 1 CopyPart-
1428759	REDUCE cov: 1202 ft: 8512 corp: 169/389Kb lim: 78749 exec/s: 1528 rss: 463Mb L: 1627/60488 MS: 1 EraseBytes-

src/test/fuzz/p2p_transport_serialization.cpp

@@ -0,0 +1,49 @@
+// Copyright (c) 2019-2020 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 <chainparams.h>
+#include <net.h>
+#include <netmessagemaker.h>
+#include <protocol.h>
+#include <test/fuzz/fuzz.h>
+
+#include <cassert>
+#include <cstdint>
+#include <limits>
+#include <optional>
+#include <vector>
+
+void initialize_p2p_transport_serialization()
+{
+    SelectParams(CBaseChainParams::REGTEST);
+}
+
+FUZZ_TARGET_INIT(p2p_transport_serialization, initialize_p2p_transport_serialization)
+{
+    // Construct deserializer, with a dummy NodeId
+    V1TransportDeserializer deserializer{Params(), (NodeId)0, SER_NETWORK, INIT_PROTO_VERSION};
+    V1TransportSerializer serializer{};
+    Span<const uint8_t> msg_bytes{buffer};
+    while (msg_bytes.size() > 0) {
+        const int handled = deserializer.Read(msg_bytes);
+        if (handled < 0) {
+            break;
+        }
+        if (deserializer.Complete()) {
+            const std::chrono::microseconds m_time{std::numeric_limits<int64_t>::max()};
+            uint32_t out_err_raw_size{0};
+            std::optional<CNetMessage> result{deserializer.GetMessage(m_time, out_err_raw_size)};
+            if (result) {
+                assert(result->m_command.size() <= CMessageHeader::COMMAND_SIZE);
+                assert(result->m_raw_message_size <= buffer.size());
+                assert(result->m_raw_message_size == CMessageHeader::HEADER_SIZE + result->m_message_size);
+                assert(result->m_time == m_time);
+
+                std::vector<unsigned char> header;
+                auto msg = CNetMsgMaker{result->m_recv.GetVersion()}.Make(result->m_command, MakeUCharSpan(result->m_recv));
+                serializer.prepareForTransport(msg, header);
+            }
+        }
+    }
+}