added MissionManifest test file

Author: Hannah Dunsmore

src/Components/MissionManifest/MissionManifest.spec.tsx

@@ -0,0 +1,10 @@
+import React from "react";
+import { render, screen } from "@testing-library/react";
+import App from "../../App";
+import MissionManifest from "./MissionManifest";
+
+test("renders learn react link", () => {
+  render(<App />);
+  const linkElement = screen.getByText(/learn react/i);
+  expect(linkElement).toBeInTheDocument();
+});