Gif: Add unit tests and fixtures to validate block content parsing

[ramonjd]

Fixes #

Changes proposed in this Pull Request:

Jetpack product discussion

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Testing instructions:

cd projects/plugins/jetpack && yarn fixtures:test extensions/blocks/gif/test

Proposed changelog entry for your changes:

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello ramonjd! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer and confirm D58071-code works as expected before merging this PR. Once this PR is merged, please commit the changes to WP.com. Thank you!
This revision will be updated with each commit to this PR

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jetpack plugin:

• Next scheduled release: April 6, 2021.
• Scheduled code freeze: March 29, 2021

[ramonjd]

This component needs refactoring before we can achieve proper test coverage.

Context: p1615166752126300-slack-CS0SC36SX