Stabilise flaky checkout spec with voucher

While applying a voucher the spec tried to proceed the checkout before
the voucher update completed rendering. It failed when the machine was
slow.

With rspec-slow-repeate I got 100% fail rate before and 100% pass after
this change.

spec/system/consumer/split_checkout_tax_incl_spec.rb

@@ -125,6 +125,7 @@ describe "As a consumer, I want to see adjustment breakdown" do
           # add Voucher
           fill_in "Enter voucher code", with: "some_code"
           click_button("Apply")
+          expect(page).to have_link "Remove code"
 
           # Choose payment
           click_on "Next - Order summary"