Improve API error UI test by checking for content instead of screenshot

[sgiehl]

Description:

The test is meant to check that no notification is shown in administration in case of an API error.
Taking a screenshot is quite useless there, as it only shows the admin overview. A notification might appear in case of a regression. Simply checking for the text content should therefor be a better choice.

Note: This test also regressed in #21605 due to the menu changes 🙈

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