test: Add tests for PCP and NATPMP implementations

Add a mock for a simple scriptable UDP server, and use this to test
various code paths (including successful mappings, timeouts and errors)
in the PCP and NATPMP implementations.

Author: laanwj

src/test/CMakeLists.txt

@@ -87,6 +87,7 @@ add_executable(test_bitcoin
   netbase_tests.cpp
   node_warnings_tests.cpp
   orphanage_tests.cpp
+  pcp_tests.cpp
   peerman_tests.cpp
   pmt_tests.cpp
   policy_fee_tests.cpp