Filtering tests based on external system availability

[jackfirth]

Fixtures are meant for integration tests, which often depend on certain machine features like network access or a Docker daemon being available. It should be possible for a fixture to indicate that the disposable resource it wraps is not available on the current machine, so that test cases using them can automatically be skipped when run in those environments. By pairing this with a disposable constructing an HTTP client for example, tests talking to AWS could automatically be skipped when run in the package catalog build server.

However, it might be better to provide this functionality as part of a more general way of conditionally skipping tests.