add testing instructions

[dutzi]

This doesn't work for me, I have a component set up like this:

@track((props) => ({
  some_prop: props.something ? 'value-1' : 'value-2',
}))
export class MyComponent extends Component {
  ...
})

[saidkholov]

The instructions are to test methods within a class. You might want to mock the library to successfully test in your case