add header tests

src/components/__tests__/header.test.js

@@ -0,0 +1,13 @@
+import React from "react"
+import renderer from "react-test-renderer"
+
+import Header from "../header"
+
+describe("Header", () => {
+  it("renders correctly", () => {
+    const tree = renderer
+      .create(<Header heading="heading" />)
+      .toJSON()
+    expect(tree).toMatchSnapshot()
+  })
+})