fix: add test case for ios13 z order bug.

The bug is fixed in: https://github.com/ecomfe/zrender/pull/522
IOS13 safari probably has a compositing bug (z order of the canvas and the consequent
dom does not act as expected) when some of the parent dom has
`-webkit-overflow-scrolling: touch;` and the webpage is longer than one screen and
the canvas is not at the top part of the page.
Check `https://bugs.webkit.org/show_bug.cgi?id=203681` for more details.