message: Improve fetch mock setup
  Move from language to mock setup file
  Add mock rule for stub image requests
diff: @@ -14,6 +14,7 @@ See the License for the specific language governing permissions and
  limitations under the License.
  */
  +import fetchMock from "fetch-mock-jest";
  import { TextDecoder, TextEncoder } from "util";
  // jest 27 removes setImmediate from jsdom
  @@ -54,3 +55,9 @@ global.TextDecoder = TextDecoder;
  // prevent errors whenever a component tries to manually scroll.
  window.HTMLElement.prototype.scrollIntoView = jest.fn();
  +
  +// set up fetch API mock
  +fetchMock.config.overwriteRoutes = false;
  +fetchMock.catch("");
  +fetchMock.get("/image-file-stub", "image file stub");
  +window.fetch = fetchMock.sandbox();
  