test: added testcase for slides snippets

[sanjuktaghosh7]

Slide UT of region tags:

• slides_text_merging
• slides_text_style_update

Fixed(https://buganizer.corp.google.com/issues/234672581)

Is it been tested?

• [x] Development testing done

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have performed a peer-reviewed with team member(s)
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] Any dependent changes have been merged and published in downstream modules