test: fix flaky WebKit cookie test

[gioboa]

What is it?

• Feature / enhancement
• Bug
• Docs / tests

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Use cases and why

• 1. One use case
• 2. Another use case

Checklist:

• My code follows the developer guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• Added new tests to cover the fix / functionality

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