refactor: prohibit direct flags access in CCoinsCacheEntry and remove invalid tests

Similarly to https://github.com/bitcoin/bitcoin/pull/30849, this cleanup is intended to de-risk https://github.com/bitcoin/bitcoin/pull/30673#discussion_r1739909068 by simplifying the coin cache public interface.

CCoinsCacheEntry provided general access to its internal flags state, even though, in reality, it could only be clean, fresh, dirty, or fresh|dirty (in the follow-up, we will remove fresh without dirty).

Once it was marked as dirty, we couldn’t set the state back to clean with AddFlags(0)—tests explicitly checked against that.

This PR refines the public interface to make this distinction clearer and to make invalid behavior impossible, rather than just checked by tests. We don't need extensive access to the internals of CCoinsCacheEntry, as many tests were simply validating invalid combinations in this way.

The last few commits contain significant test refactorings to make coins_tests easier to change in follow-ups.