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/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',