Fix migration specs using factories

We need to stub default_git_depth and default_git_depth= because some
old migrations specs try to create a record using schema before that
column was introduced.

spec/lib/gitlab/background_migration/delete_diff_files_spec.rb

@@ -5,7 +5,7 @@ describe Gitlab::BackgroundMigration::DeleteDiffFiles, :migration, :sidekiq, sch
   describe '#perform' do
     before do
       # This migration was created before we introduced ProjectCiCdSetting#default_git_depth
-      allow_any_instance_of(ProjectCiCdSetting).to receive(:default_git_depth?).and_return(true)
+      allow_any_instance_of(ProjectCiCdSetting).to receive(:default_git_depth=).and_return(0)
       allow_any_instance_of(ProjectCiCdSetting).to receive(:default_git_depth).and_return(nil)
     end
 

spec/lib/gitlab/background_migration/populate_external_pipeline_source_spec.rb

@@ -10,8 +10,8 @@ describe Gitlab::BackgroundMigration::PopulateExternalPipelineSource, :migration
     # This migration was created before we introduced metadata configs
     stub_feature_flags(ci_build_metadata_config: false)
     # This migration was created before we introduced ProjectCiCdSetting#default_git_depth
-    allow_any_instance_of(ProjectCiCdSetting).to receive(:default_git_depth?).and_return(true)
     allow_any_instance_of(ProjectCiCdSetting).to receive(:default_git_depth).and_return(nil)
+    allow_any_instance_of(ProjectCiCdSetting).to receive(:default_git_depth=).and_return(0)
   end
 
   let!(:internal_pipeline) { create(:ci_pipeline, source: :web) }

spec/migrations/remove_orphaned_label_links_spec.rb

@@ -12,8 +12,8 @@ describe RemoveOrphanedLabelLinks, :migration do
 
   before do
     # This migration was created before we introduced ProjectCiCdSetting#default_git_depth
-    allow_any_instance_of(ProjectCiCdSetting).to receive(:default_git_depth?).and_return(true)
     allow_any_instance_of(ProjectCiCdSetting).to receive(:default_git_depth).and_return(nil)
+    allow_any_instance_of(ProjectCiCdSetting).to receive(:default_git_depth=).and_return(0)
   end
 
   context 'add foreign key on label_id' do