Finish tests for get_grid_x, y, and z

Created by: mdpiper

These BMI functions use the hru_x, hru_y, and hru_elev variables in PRMS. However, there's a note on line 84 of sm_basin.f90 in the PRMS repo stating that these variables haven't been coded up yet. When they are, finish the unit tests for these BMI functions.