fuzz: add low-level target for txorphanage

ci/test/06_script_b.sh

@@ -46,6 +46,7 @@ if [ "${RUN_TIDY}" = "true" ]; then
           " src/policy/settings.cpp"\
           " src/rpc/fees.cpp"\
           " src/rpc/signmessage.cpp"\
+          " src/test/fuzz/txorphan.cpp"\
           " -p . ${MAKEJOBS} -- -Xiwyu --cxx17ns -Xiwyu --mapping_file=${BASE_BUILD_DIR}/bitcoin-$HOST/contrib/devtools/iwyu/bitcoin.core.imp"
 fi