hide menu shadow from UI tests

[sgiehl]

Description:

Every time the menu in core is changed the UI tests of this plugin might start failing, as the shadow of the menu is visible. This PR aims to hide the menu before taking the screenshot, so the shadow is no longer visible.

Review

• [ ] Functional review done
• [ ] Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• [ ] Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• [ ] Security review done
• [ ] Wording review done
• [ ] Code review done
• [ ] Tests were added if useful/possible
• [ ] Reviewed for breaking changes
• [ ] Developer changelog updated if needed
• [ ] Documentation added if needed
• [ ] Existing documentation updated if needed