Use UTC for shift times calculated in specs

ee/spec/factories/incident_management/oncall_participant.rb

@@ -16,5 +16,9 @@ FactoryBot.define do
     trait :removed do
       is_removed { true }
     end
+
+    trait :utc do
+      association :rotation, :utc, factory: :incident_management_oncall_rotation
+    end
   end
 end

ee/spec/factories/incident_management/oncall_rotations.rb

@@ -21,5 +21,9 @@ FactoryBot.define do
         create(:incident_management_oncall_participant, rotation: rotation, user: user)
       end
     end
+
+    trait :utc do
+      association :schedule, :utc, factory: :incident_management_oncall_schedule
+    end
   end
 end

ee/spec/factories/incident_management/oncall_schedules.rb

@@ -6,5 +6,9 @@ FactoryBot.define do
     sequence(:name) { |n| "On-call Schedule ##{n}" }
     description { 'On-call description' }
     timezone { 'Europe/Berlin' }
+
+    trait :utc do
+      timezone { 'Etc/UTC' }
+    end
   end
 end

ee/spec/factories/incident_management/oncall_shifts.rb

@@ -6,5 +6,9 @@ FactoryBot.define do
     rotation { participant.rotation }
     starts_at { rotation.starts_at }
     ends_at { starts_at + rotation.shift_cycle_duration }
+
+    trait :utc do
+      association :participant, :utc, :with_developer_access, factory: :incident_management_oncall_participant
+    end
   end
 end

ee/spec/workers/incident_management/oncall_rotations/persist_shifts_job_spec.rb

@@ -91,7 +91,7 @@ RSpec.describe IncidentManagement::OncallRotations::PersistShiftsJob do
     end
 
     context 'when rotation has saved shifts' do
-      let_it_be(:existing_shift) { create(:incident_management_oncall_shift) }
+      let_it_be(:existing_shift) { create(:incident_management_oncall_shift, :utc) }
       let_it_be_with_reload(:rotation) { existing_shift.rotation }
 
       context 'when current time is during a saved shift' do