Fix unit test

[shuhei]

Currently, the unit test is broken because of onclick="${() => alert('hello')}".

• bel@4 renders it as onclick="() => alert(\'hello\')"
• pelo just removes it to match the result of bel@4 for onclick=${() => alert('hello')}

[shuhei]

To be consistent with bel@4, we can render the event handler when a function is quoted with "" and remove it when it's not quoted. But I need more research on bel's behavior to figure out whether it's intentional or not.