Extract public file to maintain test schema so RSpec can use as well

Since `load_schema` is private API, we don't want it to be explicitly
called by RSpec.

This feature can be used by RSpec by exposing it in
`rails/testing/maintain_test_schema` so they don't need to reimplement
the same thing.

Author: rafaelfranca

railties/lib/rails/test_help.rb

@@ -14,20 +14,9 @@
 
 require "active_support/testing/autorun"
 
-if defined?(ActiveRecord::Base)
-  begin
-    ActiveRecord::Migration.maintain_test_schema!
-  rescue ActiveRecord::PendingMigrationError => e
-    puts e.to_s.strip
-    exit 1
-  end
-
-  if Rails.configuration.eager_load
-    ActiveRecord::Base.descendants.each do |model|
-      model.load_schema unless model.abstract_class?
-    end
-  end
+require "rails/testing/maintain_test_schema"
 
+if defined?(ActiveRecord::Base)
   ActiveSupport.on_load(:active_support_test_case) do
     include ActiveRecord::TestDatabases
     include ActiveRecord::TestFixtures

railties/lib/rails/testing/maintain_test_schema.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+if defined?(ActiveRecord::Base)
+  begin
+    ActiveRecord::Migration.maintain_test_schema!
+  rescue ActiveRecord::PendingMigrationError => e
+    puts e.to_s.strip
+    exit 1
+  end
+
+  if Rails.configuration.eager_load
+    ActiveRecord::Base.descendants.each do |model|
+      model.load_schema unless model.abstract_class?
+    end
+  end
+end