Clear detached partitions before tests run

What does this MR do and why?

Prevent flaky migration specs where detached tables were preventing some tables from being dropped

More details in https://gitlab.com/gitlab-org/gitlab/-/issues/570251#note_2777046318

References

Related to #570251

How to set up and validate locally

1. Simulate an extra detached p_duo_workflows_checkpoints partition

   I did this by changing CHECKPOINT_RETENTION_DAYS temporarily to 31 and then running RAILS_ENV=test bundle exec rails db:reset.

   You should change it back to 30 before running the spec so that it gets detached.

   Sample diff

   diff --git a/ee/app/models/ai/duo_workflows/checkpoint.rb b/ee/app/models/ai/duo_workflows/checkpoint.rb
   index 034ac6045afa1..f966bcd722c23 100644
   --- a/ee/app/models/ai/duo_workflows/checkpoint.rb
   +++ b/ee/app/models/ai/duo_workflows/checkpoint.rb
   @@ -2,7 +2,7 @@
   
    module Ai
      module DuoWorkflows
   -    CHECKPOINT_RETENTION_DAYS = 30
   +    CHECKPOINT_RETENTION_DAYS = 31
   
        class Checkpoint < ::ApplicationRecord
          include ::Ai::DuoWorkflows::SyncWorkflowAttributes

2. Run the spec: CI=1 bundle exec rspec ./ee/spec/lib/ee/gitlab/background_migration/delete_invalid_epic_issues_spec.rb

   As Rails boots, it should see the 31 partitions and detach the oldest one. Then the spec should fail.

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