add test case to /admin/customers_spec.rb to cover customer ballance, when adding new payments

[filipefurtad0]

What we should change and why (this is tech debt)

A test case should be added to /admin/customers_spec.rb cover the case when adding a new payment:

• when adding new payment on the payments section (-> choose the appropriate route to add a payment, see bundle exec rake routes)

it should:

• update the customer balance, under /admin/customers