axis_utils: Fix get_axis_cart test with Cray compiler

[J-Lentz]

Description
This modification to the get_axis_cart test is a workaround for the Cray compiler's erroneous handling of assumed-length string parameter arrays.

Fixes get_axis_cart unit test failure when built with the Cray compiler.

How Has This Been Tested?
Builds with Cray and the test passes

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

[J-Lentz]

PR closed because a fix for this bug is scheduled for CCE 17.