Fix fake date when called as function

- Apparently this just returns the toString
- Also updates tests to have `matchInlineSnapshot`
so that we're using a test oracle vs. Date.

spec/frontend/helpers/fake_date.js

@@ -15,7 +15,7 @@ export const createFakeDateClass = ctorDefault => {
     apply: (target, thisArg, argArray) => {
       const ctorArgs = argArray.length ? argArray : ctorDefault;
 
-      return RealDate(...ctorArgs);
+      return new RealDate(...ctorArgs).toString();
     },
     // We want to overwrite the default 'now', but only if it's not already mocked
     get: (target, prop) => {

spec/frontend/helpers/fake_date_spec.js

@@ -13,13 +13,17 @@ describe('spec/helpers/fake_date', () => {
     });
 
     it('should use default args', () => {
-      expect(new FakeDate()).toEqual(new Date(...DEFAULT_ARGS));
-      expect(FakeDate()).toEqual(Date(...DEFAULT_ARGS));
+      expect(new FakeDate()).toMatchInlineSnapshot(`2020-07-06T00:00:00.000Z`);
+    });
+
+    it('should use default args when called as a function', () => {
+      expect(FakeDate()).toMatchInlineSnapshot(
+        `"Mon Jul 06 2020 00:00:00 GMT+0000 (Greenwich Mean Time)"`,
+      );
     });
 
     it('should have deterministic now()', () => {
-      expect(FakeDate.now()).not.toBe(Date.now());
-      expect(FakeDate.now()).toBe(new Date(...DEFAULT_ARGS).getTime());
+      expect(FakeDate.now()).toMatchInlineSnapshot(`1593993600000`);
     });
 
     it('should be instanceof Date', () => {