TestingTorNetwork should not set AssumeReachable 1

For S55 we are trying to get self-tests and remote-tests working with IPv6. To make sure we've succeed, we need to try them with Chutney. But to try them with chutney, we need them to be enabled.

With legacy/trac#33583 (moved) we removed AssumeReachable from the chutney configurations. But that didn't actually have any effect, since AssumeReachable 1 is implicit in TestingTorNetwork 1.

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