test: add grid and codesnippet visual regression tests

[tay1orjones]

Closes #9267
Closes #9278

Adds some simple visual regression tests for CodeSnippet and the new CSS Grid implementation

Changelog

New

• grid: visual regression tests
• codesnippet: visual regression tests

Testing / Reviewing

The percy status check - click "details" link - should show new snapshots for Grid and CodeSnippet, though I'm not sure if this will require explicit "Approval" in the tool because these are new snapshots.

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