[TEST] Payment and trustline tests

- adds _payments-test.js and fixtures/_payments.js.
- covers use-cases for payment and trustlines

Part of the `_payments-test.js` do not cover the input and output between ripple-rest and rippled. These should be rewritten in the `payment-test.js` format.

I'm pushing this PR now so we can have the test coverage these tests provide though they don't fully cover their cases yet (missing input and output).

There are also some tests skipped since they deal with invalid secret we need to revisit in ripple-lib.