Right now, chutney's warnings when it can't find tor or tor-gencert suggest to set `CHUTNEY_TOR` and `CHUTNEY_TOR_GENCERT`. However, attempts to set these get overwritten by tools/test-network.sh with either a computed result based on `TOR_DIR` if that is set, or with plain program names to be searched in the path. This makes it harder to work around legacy/trac#27067.