Allow to use the latest migration in migration specs

This is useful for migration tests that relies on factories and that are
very old and/or tedious to modify to not use factories.
Signed-off-by: NRémy Coutable <remy@rymai.me>

spec/support/migrations_helpers.rb

@@ -45,7 +45,13 @@ module MigrationsHelpers
   end
 
   def migration_schema_version
-    self.class.metadata[:schema] || previous_migration.version
+    metadata_schema = self.class.metadata[:schema]
+
+    if metadata_schema == :latest
+      migrations.last.version
+    else
+      metadata_schema || previous_migration.version
+    end
   end
 
   def schema_migrate_down!