rename testcookie to make it clear it is created by Matomo #17243

inspired by https://forum.matomo.org/t/is-cookie-test-with-value-testvalue-set-by-matomo/40775/2

The cookie's name doesn't start with _pk, so it isn't obvious it is created by Matomo.

I also named the cookie _pk_testcookie_domain, so it isn't confused with this testcookie:

https://github.com/matomo-org/matomo/blob/bd7e099e46de3e10afc069624b713de4bd8df162/js/piwik.js#L3043-L3044

Review

• [ ] Functional review done
• [ ] Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• [ ] Security review done see checklist
• [ ] 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

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