test: Add jest-enzyme assertion library for better frontend tests (#10459)

* adding jest-enzyme

* enzymeify lots of assertions

* types for jest-enzyme

superset-frontend/spec/javascripts/welcome/DashboardTable_spec.tsx

@@ -56,7 +56,7 @@ describe('DashboardTable', () => {
         // there's a delay between response and updating state, so manually set it
         // rather than adding a timeout which could introduce flakiness
         wrapper.setState({ dashboards: mockDashboards });
-        expect(wrapper.find(ListView)).toHaveLength(1);
+        expect(wrapper.find(ListView)).toExist();
         done();
       });
     });