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.
test-network.sh: Use "/bin/echo -n" rather than builtin echo

The default shell on OS X is bash, which has a builtin echo. When called
in "sh" mode, this echo does not accept "-n". This patch uses "/bin/echo -n"
instead.

Partially fixes issue 13161.
Add test-network delay option

Add a --delay option to test-network.sh, which configures the delay before
the chutney network tests for data transmission. The default remains at
18 seconds if the argument isn't specified.

Apparently we should be using bootstrap status for this (eventually).

Partially implements ticket 13161.
Merge remote-tracking branches 'teor/issue-13161-test-network' and 'teor/issue-13161-TestingDirAuthVoteExit'