Correct the fallback image in PICTURE element

[mukeshpanchal27]

Summary

This PR accounts following:

• Return image/jpeg as fallback image for Picture element.
• Remove image/jpeg mime from Picture > Source and return only additional mime.
• Use share fixture for tests.
• Add/update unit test that reflect the changes.
• Reinitialize the WP_HTML_Tag_Processor so it check the additional next_tag in while loop.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: mukeshpanchal27 <mukesh27@git.wordpress.org>
Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: joemcgill <joemcgill@git.wordpress.org>
Co-authored-by: adamsilverstein <adamsilverstein@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[mukeshpanchal27]

Thanks @westonruter for the feedback. PR ready for review.

[westonruter]

Unless I'm missing something (like the issues I just commented on being addressed elsewhere) this doesn't seem quite there.

[westonruter]

It looks like $this->mock_frontend_body_hooks() needs to be added to the remaining test cases as well. Alternatively, it could be added to set_up instead.

[mukeshpanchal27]

@westonruter @adamsilverstein PR updated and ready for review.

[mukeshpanchal27]

@adamsilverstein @joemcgill Could you please review this when you have a moment? Thanks!

[joemcgill]

This is looking good.

[adamsilverstein]

Fantastic!

[adamsilverstein]

Worked well in my testing, thanks for working on this @mukeshpanchal27 🎉