Fix type mismatch error in coupler test

[J-Lentz]

Description
Fixes a type mismatch error in the coupler test, which causes a build error when the test is built with the Cray compiler.

How Has This Been Tested?
coupler tests build and pass with Cray compiler version 18 on C5.

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