Add additional embargo testing

[mark-dce]

Only one other test caught breaking permissions changes when the
GraduationJob was refactored. This adds more explicit testing in
hopes of making the desired user-facing behavior more explicit.

After Graduation, embargoed works:

• Should be discoverable (published, active, not suppressed)
• Should restrict access to ETD files (filesets restricted)
• Should have an release date the requested embargo_length after graduation
  [New Tests]
• Should be displayable (open from an access control perspective)
• Should show an embargo badge (*_restricted visibility - all, toc, or files)

[coveralls]

Coverage increased (+2.9%) to 92.081% when pulling 92b337b on postgrad_embargo into e536a99 on main.