Fix HTML validity and element finding in spec

The fixed partial was used twice on the same page. So the used element
id appeared twice on the page which is invalid HTML. It also confused
the spec.

Now we look at the actually displayed text to select the correct
element.

app/views/spree/admin/orders/_form/_adjustments.html.haml

@@ -6,7 +6,7 @@
         %tr
           %th= Spree.t('name')
           %th= Spree.t('amount')
-      %tbody#order-charges.with-border
+      %tbody.with-border
         - adjustments.each do |adjustment|
           - if (adjustment.originator_type != 'Spree::ShippingMethod') && !(adjustment.originator_type == 'Spree::TaxRate' && adjustment.amount == 0)
             %tr.total

spec/features/admin/orders_spec.rb

@@ -267,7 +267,7 @@ feature %q{
       end
 
       scenario "shows the order tax adjustments" do
-        within('tbody#order-charges') do
+        within('fieldset', text: 'LINE ITEM ADJUSTMENTS') do
           expect(page).to have_selector "td", match: :first, text: "Tax 1"
           expect(page).to have_selector "td.total", text: Spree::Money.new(10)
         end