In private networks, the defaults for some options are changed. This
means that in options_validate(), where we're testing that the defaults
are what we think they are, we fail. Use a workaround by setting a
hidden configuration option _UsingTestingTorNetwork when we have altered
the configuration this way, so that options_validate() can do the right
thing.

Fixes bug 2250, bugfix on 0.2.1.2-alpha (the version introducing private
network options).