Fix(promotions): Validate benefits on save

On Slack, we got a report that the tiered percent calculator wasn't
converting its tiers to BigDecimal when being first created. Since this
conversion happens `before_validation`, we should not turn validation
off in this controller.

Specs still run through.

promotions/lib/controllers/backend/solidus_promotions/admin/benefits_controller.rb

@@ -14,7 +14,7 @@ def new
       def create
         @benefit = @benefit_type.new(benefit_params)
         @benefit.promotion = @promotion
-        if @benefit.save(validate: false)
+        if @benefit.save
           flash[:success] =
             t("spree.successfully_created", resource: SolidusPromotions::Benefit.model_name.human)
           redirect_to location_after_save, format: :html