local_state_space_iteration_k MEX: fix bug in output

In its output, the MEX was returning values for all endogenous variables, but
it was used in a context where only the variables from oo_.dr.restrict_var_list
were expected (as is done with local_state_space_iteration_2 MEX).

This commit fixes this discrepancy, and also fixes the test that was checking
that both MEX are returning the same output.

Closes: #1768