Write integration tests

[martinm-nm]

libddog is a library that generates code meant for Datadog. In order to have confidence that our code is valid it would help enormously to have a set of integration tests that can serve as regression tests against future changes in Datadog and consequent incompatibilities in libddog.

Running the integration tests will require api keys, so they cannot be set up to run automatically in CI. They will need to be run manually.

• A dashboard that exemplifies use of all (or most of?) the widgets that libddog currently supports.
• A dashboard that uses all (or a sensible selection of?) the metrics query language, including functions.
• A test that puts these dashboards (make sure it's using dashboard id/titles in such a way that it's clear it's purely used for QA purposes) on Datadog and validates that the update was successful.
• An extension to the test that gets the dashboard after updating it and compares it to the definitions we have locally.