[REF] Move handling of default payment instrument for a payment proc…

[seamuslee001]

…essor into the create function from API and expand tests

Overview

This moves the handling for the payment_instrument_id field for payment processors out of the API layer into the BAO create function so can be used by APIv4 as well

Before

Setting payment instrument id occurs in API Layer

After

Setting payment instrument id occurs in BAO Layer

ping @colemanw @eileenmcnaughton

[civibot]

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[colemanw]

Looks great!