Reset only migration models

So that we could make sure migration tests could run even if
geo is not setup in EE.

This is because we have a model like this:

``` ruby
class Geo::BaseRegistry < ActiveRecord::Base
  def self.connection
    raise 'Geo secondary database is not configured' unless Gitlab::Geo.geo_database_configured?

    super
  end
end
```

spec/spec_helper.rb

@@ -134,13 +134,13 @@ RSpec.configure do |config|
     ActiveRecord::Migrator
       .migrate(migrations_paths, previous_migration.version)
 
-    ActiveRecord::Base.descendants.each(&:reset_column_information)
+    reset_column_in_migration_models
   end
 
   config.after(:example, :migration) do
     ActiveRecord::Migrator.migrate(migrations_paths)
 
-    ActiveRecord::Base.descendants.each(&:reset_column_information)
+    reset_column_in_migration_models
   end
 
   config.around(:each, :nested_groups) do |example|

spec/support/migrations_helpers.rb

@@ -15,6 +15,16 @@ module MigrationsHelpers
     ActiveRecord::Migrator.migrations(migrations_paths)
   end
 
+  def reset_column_in_migration_models
+    described_class.constants.sort.each do |name|
+      const = described_class.const_get(name)
+
+      if const.is_a?(Class) && const < ActiveRecord::Base
+        const.reset_column_information
+      end
+    end
+  end
+
   def previous_migration
     migrations.each_cons(2) do |previous, migration|
       break previous if migration.name == described_class.name