Rewrite unit test waitForResize helper (#4566)

Rewrite unit test waitForResize helper (#4566)

The original implementation tries to intercept events from the chart internal iframe, which ones failing on Chrome 60. Checking internals doesn't seem the best approach, instead we could consider that a chart has been resized after the resize method has been called and processed. So let's hook `Chart.resize` and callback once it's done.