Fix `insert.js` frontend test

[rhansen]

• Don't mix async functions with done callback.
• await the Promise returned from helper.waitForPromise().
• Decrease the waitForPromise() timeouts to 1s.
• Fix "hello world" comparison (lowercase "hello world" was inserted but it was looking for capitalized "Hello world").
• Fix selector for the second line (inner$('div').first() returns the first line, not the second).
• Lint: Wrap long lines.
• Delete commented-out code.

cc @JohnMcLear