Add TestingDirAuthVoteExit option (like TestingDirAuthVoteGuard)

Add the TestingDirAuthVoteExit option, a list of nodes to vote Exit for,
regardless of their uptime, bandwidth, or exit policy.

TestingTorNetwork must be set for this option to have any effect.

Works around an issue where authorities would take up to 35 minutes to
give nodes the Exit flag in a test network, despite short consensus
intervals. Partially implements ticket 13161.