Support assertion that all resources are closed

[jostick]

I really like jimfs for testing. However there are a few things that I cannot easily test at the moment, especially checking if all resources have been closed. It seems FileSystemState already contains a list of open resources, but I can't find a way to access this list.

Having a method like getOpenResources() or assertNoOpenResources() would be really nice and easy to implement.

[ronshapiro]

Does JimfsFileSystem.isOpen() suffice?

[bitzl]

I can't speak for the OP, but for our use cases JimfsFileSystem.isOpen() would be great - we could just end every relevant unit test with an assertThat(fileSystem.isOpen()).isFalse(). As far as I see, this is currently package private and therefore not accessible?