Run the chutney "mixed" network tests in multiple ways

While working on ed25519 handshake thigns, I found that the "mixed" chutney network test was often inadequate for my needs:

I wanted to try testing with clients and relays that are very old, while keeping the authorities up-to-date.
I wanted make test-network-all to run the "mixed" test multiple times, with multiple older Tor instances.

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