[Xray compatibility] Add robot-seleniumlibrary-screenshot class to render embedded images

[iliketomatoes]

When importing test results into Xray, embedded images are not rendered. Instead, Xray prints out the image's base64-encoded source as a string.

This is because Xray treats images correctly only if they include this attribute: class="robot-seleniumlibrary-screenshot".

Describe the solution you'd like
As a temporary fix, we could add the class="robot-seleniumlibrary-screenshot" attribute to each img tag here, by default.

Additional context

• See Xray docs
• See related conversation on Slack

[aaltat]

@allcontributors please add @iliketomatoes for ideas.

[allcontributors]

@aaltat

I've put up a pull request to add @iliketomatoes! 🎉