Resolve "Investigate long running examples in frontend fixture rspec"

What does this MR do?

Optimize frontend fixtures specs by using let_it_be.

This makes the frontend fixtures generation 6 minutes faster (from 15 minutes to 9 minutes):

• that's a 82% duration improvement for the fixtures generation itself.
• that's a 40% duration decrease for the frontend-fixtures job.
• frontend-fixtures-as-if-foss duration doesn't change.

Before

 Finished in 7 minutes 51 seconds (files took 49.38 seconds to load)
 85 examples, 0 failures

After

 Top 10 slowest examples (18.26 seconds, 21.2% of total time):
   Epics (JavaScript fixtures) IssuablesHelper (JavaScript fixtures) epic/mock_data.json
     4.11 seconds ./ee/spec/frontend/fixtures/epic.rb:59
   API::Issues (JavaScript fixtures) issues/related_merge_requests.json
     2.41 seconds ./spec/frontend/fixtures/issues.rb:101
   Projects::IssuesController (JavaScript fixtures) ee/issues/blocked-issue.html
     1.88 seconds ./ee/spec/frontend/fixtures/issues.rb:28
   Projects (JavaScript fixtures) ProjectsController (JavaScript fixtures) projects/overview.html
     1.72 seconds ./spec/frontend/fixtures/projects.rb:43
   Projects::MergeRequestsController (JavaScript fixtures) merge_requests/merge_request_of_current_user.html
     1.52 seconds ./spec/frontend/fixtures/merge_requests.rb:65
   Projects::MergeRequestsController (JavaScript fixtures) merge_requests/diff_comment.html
     1.39 seconds ./spec/frontend/fixtures/merge_requests.rb:85
   Projects::MergeRequests::DiffsController (JavaScript fixtures) merge_request_diffs/parallel_changes_tab_with_comments.json
     1.37 seconds ./spec/frontend/fixtures/merge_requests_diffs.rb:49
   Projects::MergeRequestsController (JavaScript fixtures) merge_requests/merge_request_with_task_list.html
     1.31 seconds ./spec/frontend/fixtures/merge_requests.rb:71
   Projects::BoardsController (JavaScript fixtures) boards/show.html
     1.28 seconds ./spec/frontend/fixtures/boards.rb:22
   Projects::MergeRequestsController (JavaScript fixtures) merge_requests/merge_request_with_comment.html
     1.26 seconds ./spec/frontend/fixtures/merge_requests.rb:91
 Top 10 slowest example groups:
   API::Issues (JavaScript fixtures)
     2.57 seconds average (2.57 seconds / 1 example) ./spec/frontend/fixtures/issues.rb:78
   Epics (JavaScript fixtures)
     2.4 seconds average (4.81 seconds / 2 examples) ./ee/spec/frontend/fixtures/epic.rb:5
   Projects::IssuesController (JavaScript fixtures)
     2.19 seconds average (2.19 seconds / 1 example) ./ee/spec/frontend/fixtures/issues.rb:5
   Projects::BlobController (JavaScript fixtures)
     1.69 seconds average (1.69 seconds / 1 example) ./spec/frontend/fixtures/blob.rb:5
   Groups::SamlProvidersController (JavaScript fixtures)
     1.64 seconds average (1.64 seconds / 1 example) ./ee/spec/frontend/fixtures/saml_providers.rb:4
   Projects::CommitController (JavaScript fixtures)
     1.34 seconds average (1.34 seconds / 1 example) ./spec/frontend/fixtures/commit.rb:5
   Projects::BoardsController (JavaScript fixtures)
     1.32 seconds average (1.32 seconds / 1 example) ./spec/frontend/fixtures/boards.rb:5
   Admin::ApplicationSettingsController (JavaScript fixtures)
     1.31 seconds average (1.31 seconds / 1 example) ./spec/frontend/fixtures/application_settings.rb:5
   Projects::DeployKeysController (JavaScript fixtures)
     1.22 seconds average (1.22 seconds / 1 example) ./spec/frontend/fixtures/deploy_keys.rb:5
   Projects::ClustersController (JavaScript fixtures)
     1.2 seconds average (1.2 seconds / 1 example) ./spec/frontend/fixtures/clusters.rb:5
 Finished in 1 minute 26.18 seconds (files took 53.53 seconds to load)
 85 examples, 0 failures

Closes #207071 (closed).