wait for slobrok server to start first
wait for slobrok server to start first

* since the SlobrokServer is started asynchronously in
  a different thread it may not be ready immediately.
  Add a loop retrying the initial connect to stabilize unit test.

vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa