[Chapel Merge] Skip memory fragmentation test under ugni for now

Branch: refs/heads/main
Revision: b5e8c96
Author: ronawho
Log Message:

Merge pull request #18059 from ronawho/skip-mem-frag-ugni

Skip memory fragmentation test under ugni for now

A bug on one of our systems is causing the test to sporadically fail, so
skip it until that is addressed.

For https://github.com/cray/chapel-private/issues/2223

Modified Files:
M test/runtime/configMatters/mem/fragmentation.skipif

Compare: https://github.com/chapel-lang/chapel/compare/77e71e6e3d37...b5e8c96c830d