Expand coverage for "system checks" (#24216)

spec/lib/admin/system_check/base_check_spec.rb

spec/lib/admin/system_check/database_schema_check_spec.rb

spec/lib/admin/system_check/elasticsearch_check_spec.rb

spec/lib/admin/system_check/media_privacy_check_spec.rb

spec/lib/admin/system_check/message_spec.rb

spec/lib/admin/system_check/rules_check_spec.rb

spec/lib/admin/system_check/sidekiq_process_check_spec.rb

spec/lib/admin/system_check_spec.rb

spec/support/examples/lib/admin/checks.rb