Add live tests to verify actual API usage.

Requires Docker and mediawiki-testwiki. Images are available at https://registry.hub.docker.com/u/blackwinter/mediawiki-testwiki/.

Status:

* Most tests from `gateway_spec.rb` have been ported.
* Some tests are currently skipped because they require additional setup.
  * [ ] Provide/import test data.
* Some tests are currently skipped because they don't match actual behaviour.
  * [ ] Fix tests, possibly using guard clauses (`required_version`).
* [ ] Add more tests, particularly investigating version-specific behaviour.
  * [ ] Test query continuation (#58).
* [ ] Add documentation (here and mediawiki-testwiki).

Result:

    Finished in 1 minute 37.12 seconds (files took 0.86261 seconds to load)
    139 examples, 0 failures, 25 pending