test: add I2P test for a runaway SAM proxy

Add a regression test for https://github.com/bitcoin/bitcoin/pull/21407.

The test creates a socket that, upon read, returns some data, but never
the expected terminator `\n`, injects that socket into the I2P code and
expects `i2p::sam::Session::Connect()` to fail, printing a specific
error message to the log.

src/test/i2p_tests.cpp

@@ -0,0 +1,44 @@
+// Copyright (c) 2021-2021 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 <i2p.h>
+#include <netaddress.h>
+#include <test/util/logging.h>
+#include <test/util/net.h>
+#include <test/util/setup_common.h>
+#include <threadinterrupt.h>
+#include <util/system.h>
+
+#include <boost/test/unit_test.hpp>
+
+#include <memory>
+#include <string>
+
+BOOST_FIXTURE_TEST_SUITE(i2p_tests, BasicTestingSetup)
+
+BOOST_AUTO_TEST_CASE(unlimited_recv)
+{
+    auto CreateSockOrig = CreateSock;
+
+    // Mock CreateSock() to create MockSock.
+    CreateSock = [](const CService&) {
+        return std::make_unique<StaticContentsSock>(std::string(i2p::sam::MAX_MSG_SIZE + 1, 'a'));
+    };
+
+    CThreadInterrupt interrupt;
+    i2p::sam::Session session(GetDataDir() / "test_i2p_private_key", CService{}, &interrupt);
+
+    {
+        ASSERT_DEBUG_LOG("Creating SAM session");
+        ASSERT_DEBUG_LOG("too many bytes without a terminator");
+
+        i2p::Connection conn;
+        bool proxy_error;
+        BOOST_REQUIRE(!session.Connect(CService{}, conn, proxy_error));
+    }
+
+    CreateSock = CreateSockOrig;
+}
+
+BOOST_AUTO_TEST_SUITE_END()