19897: Add tests for writing immutable mount objects

19897: Add tests for writing immutable mount objects

I originally wrote these expecting to get them to return ENOTSUP as
well. However, that doesn't happen now because of how the Operations
class calls Directory.writable() and turns False into EPERM. I'm
committing the tests since they're useful as-is, with a note that I'd
like to convert them to ENOTSUP in the future.

Arvados-DCO-1.1-Signed-off-by: Brett Smith <brett.smith@curii.com>