TOR_DIR shouldn't override CHUTNEY_TOR and CHUTNEY_TOR_GENCERT
Right now, chutney's warnings when it can't find tor or tor-gencert suggest to set
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 (moved).