[test] Add CCoinsViewCache Access/Modify/Write tests

Add more comprehensive unit tests for CCoinsViewCache. Right now it is hard to
refactor caching code or fix bugs in the caching logic because you have to try
to mentally enumerate all the different states the cache might be in to make
sure a change doesn't cause unintended consequences. The new tests explicitly
enumerate relevant cache states, documenting and verifying the behavior in each
state, so it will be safer and easier to make changes to the caching code in
the future.

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