Image pasted during tests has no alternative

Automated accessibility tests have highlighted an issue with our test suite.
It seems like this issue is low priority because it's just that we don't provide accessible test content when testing, fixing it won't improve use experience but just test quality.

```
Validation in the test method [pasteAndEditExternalImage]
Check for [org.xwiki.test.ui.po.ViewPage] at http://xwikiweb:8080/xwiki/bin/view/NestedImageIT/pasteAndEditExternalImage/WebHome.
Found [1] items

1: Images must have alternate text
Description: Ensures <img> elements have alternate text or a role of none or presentation
Help URL: https://dequeuniversity.com/rules/axe/4.8/image-alt?application=axeAPI
Help: Images must have alternate text
Impact: critical
Tags: cat.text-alternatives, wcag2a, wcag111, section508, section508.22.a, TTv5, TT7.a, TT7.b, EN-301-549, EN-9.1.1.1, ACT

HTML element:
<img src="http://xwikiweb:8080/xwiki/bin/download/NestedImageIT/pasteAndEditExternalImage/WebHome/image.gif">
Selector: [p > img]
Fix any of the following:
Element does not have an alt attribute
aria-label attribute does not exist or is empty
aria-labelledby attribute does not exist, references elements that do not exist or references elements that are empty
Element has no title attribute
Element's default semantics were not overridden with role="none" or role="presentation"
```