Should SystemPropertyExtension use closest or enclosing annotations

In stream and while discussing (the already merged) #187 we were thinking about following thing:

At the moment the SystemPropertyExtension only looks for the closest annotation. When @nicolaiparlog was doing #187 he kept it that way to keep the tests passing, but we both asked ourself if that’s the correct way the extension should work or if it should also take the enclosing annotations into account.

Note: The current docs say to only use closest, but of course they “only” fit to current implementation.

[Hancho2009]

This also effects EnvironmentVariableExtension because both extensions should probably operate equally in that regards #180

[beatngu13]

AbstractEntryBasedExtension probably should serve as a central place to control the annotation processing, like I said here:

Note that subclasses of AbstractEntryBasedExtension are currently responsible for discovering their own annotations. In a second step, we might move additional code into AbstractEntryBasedExtension. However, for now I think the way it is implemented is already a good improvement.

This way, we get consistent behavior for all subclasses such as SystemPropertyExtension and EnvironmentVariableExtension.

I am not sure if I would go for stuff like @IgnoreSetOrClearSystemProperty. This can make things really complicated, both from a maintainer and a user perspective. And one can easily achieve the desired behavior with what we already have.

I would suggest to look into similar extensions in Jupiter and—if applicable—try to copy their behavior, such that users get consistent behavior across the JUnit universe.

Thoughts?