E2E for sharing a snippet

What does this MR do?

Adds an end-to-end to test sharing of snippets via Embed/Share dropdown.

It covers the following scenarios:

• Public snippet will display a sharing link and it can be shared with non-signed in users
• When visibility is changed to Private, Embed/Share dropdown with links will disappear

Closes gitlab-org/quality/testcases#985 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content