Merge branch '23034-memoize-ar-migrator-migrations-in-tests' into 'master'

Memoize ActiveRecord::Migrator.migrations in tests

See merge request !6723

config/initializers/ar_speed_up_migration_checking.rb

+if Rails.env.test?
+  require 'active_record/migration'
+
+  module ActiveRecord
+    class Migrator
+      class << self
+        alias_method :migrations_unmemoized, :migrations
+
+        # This method is called a large number of times per rspec example, and
+        # it reads + parses `db/migrate/*` each time. Memoizing it can save 0.5
+        # seconds per spec.
+        def migrations(paths)
+          @migrations ||= migrations_unmemoized(paths)
+        end
+      end
+    end
+  end
+end