test: make sure to return promise from test (#5336)

Author: SimenB

packages/jest-runtime/src/__tests__/runtime_jest_fn.js

@@ -47,7 +47,7 @@ describe('Runtime', () => {
   });
 
   describe('jest.clearAllMocks', () => {
-    it('clears all mocks', () => {
+    it('clears all mocks', () =>
       createRuntime(__filename).then(runtime => {
         const root = runtime.requireModule(runtime.__mockRootPath);
 
@@ -64,7 +64,6 @@ describe('Runtime', () => {
 
         expect(mock1).not.toBeCalled();
         expect(mock2).not.toBeCalled();
-      });
-    });
+      }));
   });
 });