test: add a test that -i2pacceptincoming=0 creates a transient session

The test is a bit primitive as it checks the Bitcoin Core log and
assumes that if it logs that it creates a transient session, then it
does that indeed.

A more thorough test would be to check that it indeed sends the
`SESSION CREATE ... DESTINATION=TRANSIENT` command and that it uses
the returned I2P address for connecting, even for repeated connections
to the same I2P peer. That would require a mocked SAM server (proxy)
implementation in Python.

test/functional/p2p_i2p_sessions.py

@@ -0,0 +1,36 @@
+#!/usr/bin/env python3
+# Copyright (c) 2022-2022 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""
+Test whether persistent or transient I2P sessions are being used, based on `-i2pacceptincoming`.
+"""
+
+from test_framework.test_framework import BitcoinTestFramework
+
+
+class I2PSessions(BitcoinTestFramework):
+    def set_test_params(self):
+        self.num_nodes = 2
+        # The test assumes that an I2P SAM proxy is not listening here.
+        self.extra_args = [
+            ["-i2psam=127.0.0.1:60000", "-i2pacceptincoming=1"],
+            ["-i2psam=127.0.0.1:60000", "-i2pacceptincoming=0"],
+        ]
+
+    def run_test(self):
+        addr = "zsxwyo6qcn3chqzwxnseusqgsnuw3maqnztkiypyfxtya4snkoka.b32.i2p"
+
+        self.log.info("Ensure we create a persistent session when -i2pacceptincoming=1")
+        node0 = self.nodes[0]
+        with node0.assert_debug_log(expected_msgs=[f"Creating persistent SAM session"]):
+            node0.addnode(node=addr, command="onetry")
+
+        self.log.info("Ensure we create a transient session when -i2pacceptincoming=0")
+        node1 = self.nodes[1]
+        with node1.assert_debug_log(expected_msgs=[f"Creating transient SAM session"]):
+            node1.addnode(node=addr, command="onetry")
+
+
+if __name__ == '__main__':
+    I2PSessions().main()

test/functional/test_runner.py

@@ -328,6 +328,7 @@ BASE_SCRIPTS = [
     'feature_blocksdir.py',
     'wallet_startup.py',
     'p2p_i2p_ports.py',
+    'p2p_i2p_sessions.py',
     'feature_config_args.py',
     'feature_presegwit_node_upgrade.py',
     'feature_settings.py',