mock Date.now so persistance tests don't rely on actual time (needed for updatedAt and snapshot testing)

Author: pieh

packages/gatsby/src/redux/__tests__/__snapshots__/index.js.snap

@@ -64,7 +64,7 @@ Object {
       "path": "/my-sweet-new-page/",
       "pluginCreatorId": "",
       "pluginCreator___NODE": "",
-      "updatedAt": 1606471951096,
+      "updatedAt": 1,
     },
   },
   "pendingPageDataWrites": Object {

packages/gatsby/src/redux/__tests__/index.js

@@ -91,6 +91,12 @@ describe(`redux db`, () => {
   const initialComponentsState = _.cloneDeep(store.getState().components)
 
   beforeEach(() => {
+    // mock Date.now so Date.now() doesn't change in between tests
+    const RealDateNow = Date.now
+    let DateNowCallCount = 0
+    // simulate passage of time by increasing call counter (instead of actual time value)
+    Date.now = jest.fn(() => ++DateNowCallCount)
+
     store.dispatch(
       createPage(
         {
@@ -107,6 +113,8 @@ describe(`redux db`, () => {
       )
     )
 
+    Date.now = RealDateNow
+
     writeToCache.mockClear()
     mockWrittenContent.clear()
   })
@@ -133,6 +141,8 @@ describe(`redux db`, () => {
     // make sure data was read and is not the same as our clean redux state
     expect(data.components).not.toEqual(initialComponentsState)
 
+    // we ignore pages in this assertion, because it contains `updatedAt` timestamp field that
+    // will change depending on time tests were executed
     expect(data).toMatchSnapshot()
   })