[next] Support server side jest testing (#1747)

* Support server side jest testing

* feat: more tests

src/core/client/test/setup.ts

@@ -0,0 +1,24 @@
+import "./enzyme";
+import "./jsdom";
+
+// TODO: Remove when fixed.
+// Mock React.createContext because of https://github.com/airbnb/enzyme/issues/1509.
+function mockReact() {
+  const originalReact = require.requireActual("react");
+  return {
+    ...originalReact,
+    createContext: jest.fn(defaultValue => {
+      let value = defaultValue;
+      const Provider = (props: any) => {
+        value = props.value;
+        return props.children;
+      };
+      const Consumer = (props: any) => props.children(value);
+      return {
+        Provider,
+        Consumer,
+      };
+    }),
+  };
+}
+jest.mock("react", () => mockReact());