fix flaky integration tests (login helper?)

[yksflip]

 1) Order balancing charges correct amounts creates financial transaction with correct amount
    Failure/Error: expect(goa.result).to be > 0

      expected: > 0
           got:   0
    # ./spec/models/order_spec.rb:177:in `block (3 levels) in <top (required)>'

  2) product distribution agrees to documented example
     Failure/Error: expect(oa.quantity).to eq(6)

       expected: 6
            got: 5

       (compared using ==)
     # ./spec/integration/product_distribution_example_spec.rb:50:in `block (2 levels) in <top (required)>'

[yksflip]

look's like the login method in session_helper.rb is flaky.
Here parts of the entered password ended up in the username field ... pretty weird. Maybe adding a wait: could help...