fix hr.attendance creation

in v7, the field sheet_id of hr.attendance is a function field, so we cannot
write to it.
However, passing a sheet_id key in the context at create time helps the search
a bit.
test timesheet is created with correct values

this required changing the employee, because of
https://github.com/odoo/odoo/issues/2011 which makes employee_fp unsuitable for
testing, and fixing the user used to run the wizard in the test (because the
uid is used to find the timesheet_sheet in the hr.attendance sheet_id function
field).