add subnav tests

.eslintrc.js

@@ -3,7 +3,7 @@ module.exports = {
     browser: true,
     es2021: true,
     node: true,
-    jest: true
+    jest: true,
   },
   extends: [
     "next/core-web-vitals",

src/components/SubNav/SubNav.test.tsx

@@ -0,0 +1,21 @@
+import React from "react"
+import { render, screen, act } from "@testing-library/react"
+import userEvent from "@testing-library/user-event"
+import mockRouter from "next-router-mock"
+// this import, as well as its use on line 15 is to avoid the following error:
+// TypeError: Cannot use 'in' operator to search for 'beforePopState' in null
+import { MemoryRouterProvider } from "next-router-mock/MemoryRouterProvider"
+
+import SubNav from "./SubNav"
+
+jest.mock("next/router", () => jest.requireActual("next-router-mock"))
+
+describe("SubNav", () => {
+  it("sends you to Subject Heading Explorer", async () => {
+    render(<SubNav activePage="search" />, {
+      wrapper: MemoryRouterProvider,
+    })
+    const subNavLinks = screen.getAllByRole("link")
+    expect(subNavLinks).toHaveLength(3)
+  })
+})