[tor-bugs] #21903 [Core Tor/Chutney]: Disable DNS in chutney by default, and add an option to enable it

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#21903: Disable DNS in chutney by default, and add an option to enable it
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     Reporter:  teor              |      Owner:  teor
         Type:  defect            |     Status:  new
     Priority:  Medium            |  Milestone:
    Component:  Core Tor/Chutney  |    Version:
     Severity:  Normal            |   Keywords:
Actual Points:                    |  Parent ID:
       Points:  0.5               |   Reviewer:
      Sponsor:                    |
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 Due to #21900, we need to explicitly specify `ServerDNSResolvConfFile
 /dev/null` for chutney to work on macOS when the network is unavailable.

 We should also set `ServerDNSDetectHijacking 0`.

 This should be the default, because:
 * users who run chutney might not want it using DNS in a detectable
 pattern, and
 * it makes chutney more reliable, because it no longer depends on a
 working DNS.

 Some users will want chutney to be able to use hostnames, so we should add
 a tools/test-network.sh option and environmental variable to re-enable the
 default ServerDNSResolvConfFile setting (or, even better, use a custom
 ServerDNSResolvConfFile).

 There seems to be no reason to turn on ServerDNSDetectHijacking: some
 users might be using chutney with internal DNS names.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/21903>
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