Stabilise enterprise fee spec

The problem here was that the second fee was created in a `let!` block
that came after a `before` block which visited the page. In some cases
it worked but sometimes the fee wasn't created yet when the page was
loaded. Form changes in the second row were not affecting the second fee
as intended.

spec/system/admin/enterprise_fees_spec.rb

@@ -161,6 +161,7 @@ describe '
       let!(:fee1) { create(:enterprise_fee, fee_type: "sales", enterprise_id: enterprise.id) }
 
       before do
+        visit admin_enterprise_fees_path
         # edits the existing fee
         select 'Fundraising', from: "#{prefix}_fee_type"
         fill_in "#{prefix}_name", with: 'Hello!'