fix: Pass errors to done in Jest.

Author: cartant

fixtures/jest/passing-spec.ts

@@ -76,7 +76,6 @@ describe("marbles", () => {
       m.flush();
 
       expect(done).toBeInstanceOf(Function);
-      expect(done.fail).toBeInstanceOf(Function);
       setTimeout(done, 0);
     })
   );

source/jest/observe.ts

@@ -6,16 +6,13 @@
 import { Observable } from "rxjs";
 import { DoneSubscriber } from "../done-subscriber";
 
-export interface DoneFunction {
-  (): void;
-  fail: (error: any) => void;
-}
+export type DoneFunction = (error?: Error) => void;
 
 export function observe<T>(
   observableTest: () => Observable<T>
 ): (done: DoneFunction) => void {
   return (done: DoneFunction) => {
-    const subscriber = new DoneSubscriber(done.fail, done);
+    const subscriber = new DoneSubscriber(done, done);
     observableTest().subscribe(subscriber);
   };
 }