Add tests for the storage description requirements

[edwardsph]

• https://solidproject.org/ED/protocol#server-storage-description - Test that GET, HEAD, OPTIONS requests to any resource return a link header targeting the storage description resource:
• https://solidproject.org/ED/protocol#server-storage-description-resource - Test that the storage description resource contains statements. At present the spec as ambiguous about what for the resource takes and what it contains so the test would simply check that it the resolves. See Server Description solid/specification#355 (comment)