overlay: test cases that force read-only mount

Verify that overlay is mounted read-only and that it cannot
be remounted rw.

- Mount with no upper dir
- Failure to create work dir

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>
3 files changed