chutney should try harder to find a stable/different version of tor

Currently, chutney tries to find a stable version of tor under tor-stable anywhere in the path, then stops.

We could:

Preserve the old path before test-network.sh modifies it, and use the first tor in that path
Check for /usr/bin/tor and /usr/local/bin/tor (And maybe /opt ?)
use each version in a round-robin fashion (Split to #17014 (moved), child of #16954 (moved))

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