fix: use unresolvable domain for tests

backbone.com is a real domain owned by someone else
Protect against accidentally sending requests to their
domain if the API mockers are not working.

fixtures/config-invalid.json

 {
-  "api": "https://example.backbone.com/api",
+  "api": "https://example.backbone.fake/api",
   "client_id": "invalid-client-id",
   "client_name": "InvalidClientName",
   "subscription": "dot.delimited.topic.subscription.#",

fixtures/config-valid.json

 {
-  "api": "https://example.backbone.com/api",
+  "api": "https://example.backbone.fake/api",
   "client_id": "unique-client-id",
   "client_name": "UniqueClientName",
   "subscription": "dot.delimited.topic.subscription.#",