Speed up embargo tests

[mark-dce]

Previously, the embargo tests were running expensive setup once for
test example. This change refactors the setup to only run once per
test group. Overall, this results in over 50% faster testing time.

BEFORE

% bundle exec rspec ./spec/system/embargo_show_spec.rb
Run options: exclude {:smoke_test=>true}

Randomized with seed 38550
.....

Top 5 slowest examples (396.2 seconds, 100.0% of total time):
  Display an ETD with embargoed content viewed by depositor pre-graduation
    93.76 seconds ./spec/system/embargo_show_spec.rb:62
  Display an ETD with embargoed content viewed by unauthenticated user post-graduation
    89.25 seconds ./spec/system/embargo_show_spec.rb:138
  Display an ETD with embargoed content viewed by approver with Versions enabled
    84.82 seconds ./spec/system/embargo_show_spec.rb:109
  Display an ETD with embargoed content viewed by a school approver post-graduation
    64.39 seconds ./spec/system/embargo_show_spec.rb:121
  Display an ETD with embargoed content viewed by approver pre-graduation
    63.98 seconds ./spec/system/embargo_show_spec.rb:87

Finished in 6 minutes 36 seconds (files took 15.28 seconds to load)
5 examples, 0 failures

Randomized with seed 38550

AFTER

% bundle exec rspec ./spec/system/embargo_show_spec.rb --seed 38550
Run options: exclude {:smoke_test=>true}

Randomized with seed 38550
.....

Top 5 slowest examples (72.11 seconds, 54.1% of total time):
  Display an ETD with embargoed content post graduation
    22.21 seconds ./spec/system/embargo_show_spec.rb:112
  Display an ETD with embargoed content viewed by approver pre-graduation
    15.97 seconds ./spec/system/embargo_show_spec.rb:83
  Display an ETD with embargoed content viewed by depositor pre-graduation
    13.81 seconds ./spec/system/embargo_show_spec.rb:61
  Display an ETD with embargoed content base etd has expected embargo values after creation
    10.12 seconds ./spec/system/embargo_show_spec.rb:47
  Display an ETD with embargoed content viewed by approver with Versions enabled
    10 seconds ./spec/system/embargo_show_spec.rb:102

Finished in 2 minutes 13.3 seconds (files took 15.44 seconds to load)
5 examples, 0 failures

Randomized with seed 38550

[coveralls]

Coverage remained the same at 92.023% when pulling 18ecff6 on before_all into 5b9807d on main.