[jest] use default type params for mock and spy declarations (#20577)

types/jest/index.d.ts

@@ -9,7 +9,7 @@
 //                 Ika <https://github.com/ikatyang>
 //                 Waseem Dahman <https://github.com/wsmd>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
-// TypeScript Version: 2.2
+// TypeScript Version: 2.3
 
 declare var beforeAll: jest.Lifecycle;
 declare var beforeEach: jest.Lifecycle;
@@ -488,12 +488,12 @@ declare namespace jest {
         new (...args: any[]): any;
     }
 
-    interface Mock<T> extends Function, MockInstance<T> {
+    interface Mock<T = {}> extends Function, MockInstance<T> {
         new (...args: any[]): T;
         (...args: any[]): any;
     }
 
-    interface SpyInstance<T> extends MockInstance<T> {
+    interface SpyInstance<T = {}> extends MockInstance<T> {
         mockRestore(): void;
     }