Reset fixture between suites

[caddoo]

Description

Fix PrivacyManager UI tests when running under --plugin

This change fixes a UI test state leak that showed up when PrivacyManager was run as an isolated plugin job in CI.

Why it happened:

• The screenshot test runner treats PrivacyManager and --plugin=PrivacyManager differently.
• Passing the suite name alone runs the matching suite, but --plugin=PrivacyManager switches the runner into plugin mode and reuses persisted fixture state differently.
• Under --persist-fixture-data, that let PrivacyManager settings from previous tests carry into later runs, which caused Anonymize Order ID and Anonymize Referrer to appear in the wrong state.

How to reproduce:

• ddev matomo:console tests:run-ui PrivacyManager --persist-fixture-data
• ddev matomo:console tests:run-ui --plugin=PrivacyManager --persist-fixture-data

What changed:

• Reset PrivacyManager instance-level settings and site-specific overrides before and after the affected UI suites.
• This keeps the screenshot tests self-contained even when fixture data is persisted across runs.

Why this matters:

• CI is being split to target individual plugins, so these tests need to behave correctly when PrivacyManager runs in isolation instead of as part of a larger shared UI batch.

Checklist

• [✔] I have understood, reviewed, and tested all AI outputs before use
• [✔] All AI instructions respect security, IP, and privacy rules

Review

• Functional review done
• Potential edge cases thought about (behaviour 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
• New documentation added or existing documentation updated