Run some steps in parallel

- `_takeScreenshot()` and `_findElements()` in `Browser.prototype.captureState()`
- `getClientRect()` and `getComputedCss()` in `element-rect.get()`
- `getLocation()` and `getSize()` in `Element.prototype.getClientRect()`

This should speed-up tests a little.

/cc @SevInf