When invoking the `Create()` and `Delete()` methods on
`MockDirectoryInfo`, the `Exists` property should intrinsically invoke
`Refresh()` the next time it is called. This is necessary to prevent
returning stale state about the directory after those operations.

Additional test cases have been added for `MoveTo()` as well, to ensure
it also not returning stale state.

For context, this change was motivated by PR TestableIO#828 where similar issues
were solved in `MockFileInfo`.