Allow skill owners to bring their own Azure subscription for integration tests

As of today, all the integration tests run against one shared test subscription. This won't scale as we add more tests that need access to Azure resources. Once we support skill owners bringing their own test subscriptions, we can adopt the following model:

1. Run no-Azure tests and core tests in the shared test subscription, such as skill invocation.
2. Skill owners opt in to more expensive test cases, bring their own test subscription and configure the tests to run in those subscriptions.

Start by locating the integration-test configuration and the setup for the shared Azure test subscription. Determine how no-Azure, core, and opt-in subscription-backed tests should be separated, and document what skill owners must configure for their own subscriptions.

Written by the indexing model from the issue text.