Re-enable foriegn key tests on MySQL

This reverts commit e84799d9, e31104c1 and e6ca8e2c

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -936,7 +936,6 @@ def test_reflect_the_most_recent_change
   end
 end
 
-unless current_adapter?(:MysqlAdapter, :Mysql2Adapter)
 class BelongsToWithForeignKeyTest < ActiveRecord::TestCase
   def test_destroy_linked_models
     address = AuthorAddress.create!
@@ -945,4 +944,3 @@ def test_destroy_linked_models
     author.destroy!
   end
 end
-end

activerecord/test/schema/schema.rb

@@ -84,9 +84,7 @@ def except(adapter_names_to_exclude)
   create_table :author_addresses, force: true do |t|
   end
 
-  unless current_adapter?(:MysqlAdapter, :Mysql2Adapter)
-    add_foreign_key :authors, :author_addresses
-  end
+  add_foreign_key :authors, :author_addresses
 
   create_table :author_favorites, force: true do |t|
     t.column :author_id, :integer