test: check replaced iff translatable comment

All translatable comments should be replaced, but nothing else.

curriculum/test/utils/extract-js-comments.test.js

@@ -0,0 +1,25 @@
+/* global expect */
+const extractJSComments = require('./extract-js-comments');
+
+const someJS = `
+// single line comment
+
+/*
+a multiline comment
+*/
+
+var x = 'y'; // single line comment
+
+var y = '// single line comment';
+
+`;
+
+describe('extractJSComments', () => {
+  it('should return an object with comment keys and count values', () => {
+    const commentCounts = {
+      'single line comment': 2,
+      'a multiline comment': 1
+    };
+    expect(extractJSComments(someJS)).toEqual(commentCounts);
+  });
+});