Fix EE specific relationship and tests

Fixing EE relationsip

ee/app/models/ee/merge_request.rb

@@ -79,7 +79,7 @@ module EE
     end
 
     def get_on_train!(user)
-      create_merge_train!(user: user)
+      create_merge_train!(user: user, target_project: target_project, target_branch: target_branch)
     end
 
     def get_off_train!

ee/app/models/ee/project.rb

@@ -62,7 +62,7 @@ module EE
       accepts_nested_attributes_for :software_license_policies, allow_destroy: true
       has_many :packages, class_name: 'Packages::Package'
       has_many :package_files, through: :packages, class_name: 'Packages::PackageFile'
-      has_many :merge_trains
+      has_many :merge_trains, foreign_key: 'target_project_id', inverse_of: :target_project
 
       has_many :sourced_pipelines, class_name: 'Ci::Sources::Pipeline', foreign_key: :source_project_id
 

ee/app/models/merge_train.rb

 # frozen_string_literal: true
 
 class MergeTrain < ApplicationRecord
+  belongs_to :target_project, class_name: "Project"
   belongs_to :merge_request
   belongs_to :user
   belongs_to :pipeline, class_name: 'Ci::Pipeline'

ee/spec/factories/merge_trains.rb

@@ -2,6 +2,8 @@
 
 FactoryBot.define do
   factory :merge_train do
+    target_branch 'master'
+    target_project factory: :project
     merge_request
     user
     pipeline factory: :ci_pipeline