Fix db dir in Schema unit test

spec/migrations/active_record/schema_spec.rb

@@ -7,7 +7,7 @@ require 'spec_helper'
 
 RSpec.describe ActiveRecord::Schema, schema: :latest do
   let(:all_migrations) do
-    migrations_directories = %w[db/migrate db/post_migrate].map { |path| Rails.root.join(path).to_s }
+    migrations_directories = Rails.application.paths["db/migrate"].paths.map(&:to_s)
     migrations_paths = migrations_directories.map { |path| File.join(path, '*') }
 
     migrations = Dir[*migrations_paths] - migrations_directories