Take nested attachments into account when doing the screenshots diff

[nonatomiclabs]

Context

The screen-diff plugin works great but it doesn't take into account attachments that are nested in an Allure step, or that are attached to the before-/after-test stages. This PR extends the findImage function of the plugin to take all the steps/stages into account.

I tested it locally, as can be seen on the following screenshot (black boxes hide private information, red lines highlighting the change):

Checklist

• Sign Allure CLA
• Provide unit tests: not sure how to add them, as I didn't find tests for this plugin

[baev]

Screen Diff plugin is deprecated. We suggest you use imagediff attachments instead. See #1145 for details

[nonatomiclabs]

Thanks a lot, I'll have a look at imagediff!