* don't use retry to try restarting the agent
this caused some issues when the startup would fail in
a separate go routine
* clear out the data directory on every retry since the ports
are stored in the raft data files
* set a unique id for every agent to allow for tracking of
concurrent output
This brings down the test run from 108 sec to 15 sec.
There is an occasional port conflict because of the nature
the next port is chosen. So far it seems rare enough to live
with it.
TestAgent will replace the following mechanisms to
start test agents in subsequent requests:
* makeAgentXXX
* makeDNSServerXXX
* makeHTTPServerXXX
* testServer
* httpTest