payment_method_spec: avoid INSERT in before(:all)

This before(:all) block (which is outside of the database transaction that specs are run in) was creating four payment methods which caused order_spec to fail occasionally (to reproduce: run rspec in core a few times).

```
Failures:

  1) Spree::Order#available_payment_methods does not include a payment method twice if display_on is blank
     Failure/Error: expect(order.available_payment_methods.count).to eq(1)

       expected: 1
            got: 4

       (compared using ==)
     # ./spec/models/spree/order_spec.rb:855:in `block (3 levels) in <top (required)>'
```

This changes it to a normal before block.