fix: add test case for ios13 z order bug.

The bug is fixed in: ecomfe/zrender#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.

Author: 100pah

test/-cases.js

@@ -33,6 +33,7 @@
             'touch-test.html',
             'touch-candlestick.html',
             'tooltip-touch.html',
+            'ios13-z-bug.html',
             'bar.html',
             'line.html',
             'geoLine.html',