add allocation checks to horiz_interp_type_eq

[rem1776]

Description
Adds allocation checks for horiz_interp_type_eq before it assigns any allocatable fields.

Also adds some assignments to the test to reproduce the GNU errors from before these changes were made and cleans up some unused variables.

Fixes #1557

How Has This Been Tested?
gcc 13 and oneapi 2024.2 on the amd box

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