Fix output_yaml tests

[J-Lentz]

Description
This fixes an issue where the filename argument of write_yaml_from_struct_3 is passed as a Fortran-style string, when a null-terminated string is expected. When the test is built with the Cray compiler, this issue causes test.yaml to be created with junk characters at the end of the filename, which causes a "file not found" error when the test subsequently attempts to read the file.

How Has This Been Tested?
output_yaml test builds, runs, and passes with CCE 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

[uramirez8707]

c_null_char strikes again!