everything in the C4::Circulation really requires items, so let's do this in the setup phase.
named parameters: borrower => borrower hashref, computed from $self->{'memberid'} if not given barcode => item barcode, barcode of $self->{'items'}[0] if not given issuedate => YYYY-MM-DD of date to mark issue checked out. defaults to today.
pass in an itemnumber, returns a barcode.
Should this get moved up to KohaTest.pm? Or, is there a better alternative in C4?