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

• [x] Functional review done
• [x] Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• [x] Security review done see checklist
• [x] Code review done
• [x] Tests were added if useful/possible
• [x] Reviewed for breaking changes
• [x] Developer changelog updated if needed
• [x] Documentation added if needed
• [x] Existing documentation updated if needed

build js

build js