test: add --remote option, to test against a real DB4S end point

By default (without this option), the test spin up a local "mock"
server to test the HTTPS API calls.

With the --remote option however, the local mock server isn't
started and the HTTPS API calls are done using the url specified.

  $ go test --check.v --remote https://docker-dev.dbhub.io:5550

This is because these tests are likely to be useful for the
development of the DB4S end point code too, as they should help
catch any accidental API breakage.

Author: justinclift