Wait for juttle-engine to start in each unit test.
Wait for juttle-engine to start in each unit test.

While running this locally, I noticed races where juttle-engine would
be started but the http server wasn't accepting connections yet,
leading to errors like:

Error: connect ECONNREFUSED 127.0.0.1:10000
    at Object.exports._errnoException (util.js:874:11)
    at exports._exceptionWithHostPort (util.js:897:20)
    at TCPConnectWrap.afterConnect [as oncomplete] (net.js:1063:14)

Fix this by waiting until a client can fetch /api/v0/version-info and
get an 200. Use bluebird-retry to retry the fetch.

juttle/juttle-engine,juttle/juttle-engine