Change CodeEditor test to use a snapshot test

We only care that the component renders without an error and doesn't
change unintentionally, which makes this a perfect use case for snapshot
testing.

components/code-editor/test/__snapshots__/index.js.snap

@@ -0,0 +1,7 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`CodeEditor should render without an error 1`] = `
+<textarea
+  value="<b>wowee</b>"
+/>
+`;

components/code-editor/test/index.js

@@ -18,6 +18,6 @@ describe( 'CodeEditor', () => {
 		} ) );
 
 		const wrapper = shallow( <CodeEditor value={ '<b>wowee</b>' } /> );
-		expect( wrapper.find( 'textarea' ) ).toHaveLength( 1 );
+		expect( wrapper ).toMatchSnapshot();
 	} );
 } );