test-network.sh: use TOR_DIR as fallback instead of first priority

If CHUTNEY_TOR or CHUTNEY_TOR_GENCERT have already been set explicitly by environment variable or command-line option, the previous behavior of overriding that using TOR_DIR is surprising; especially if TOR_DIR itself was implicitly set to ../tor.

e.g. if there happens to be a tor source directory at ../tor, past behavior of test-network.sh --tor=/preferred/tor was to ignore the command-line flag and attempt to use ../tor/src/app/tor. The new behavior is to use /preferred/tor.