fix array indices for horiz_interp_type assignment test

[rem1776]

Description
Fixes an invalid assignment in the assignment test where it was iterating from 1 to the length of the array when it should have been iterating over the compute domain bounds when taking midpoints. This fixes gcc errors with bounds checking turned on.

How Has This Been Tested?
gcc 13 on amd box with -fcheck=all

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• make distcheck passes