e2e: add test specs for `oras manifest delete` command

[qweeah]

Happy paths:

• should delete a manifest with interactive confirmation, with delete prompt
• should delete a manifest with confirmation flag
• should delete a manifest with confirmation flag and output descriptor to stdout
• progress status should be disabled when --descriptor is provided
• should show preview hint in help doc

Bad paths:

• should fail if no blob reference is provided
• should fail if no confirmation flag and descriptor flag is provided
• should fail directly if not logged in