Some of the testcases in bin/ls/, e.g. bin/ls/ls_tests:S_flag , allocate large sparse files in order to test out prefixes and sorting. Unfortunately, if the files cannot be allocated because the perceived file is larger than the amount of reservable memory, the testcases will fail.
The testcases need to do more intelligent requirements checking, depending on the filesystem type.
This is an unfortunate tmpfs limitation; it would be nice to support sparse files larger than the filesystem.