Right now TEST_LIST targets such as build/unittests.txt depend on all of the tests in the list, meaning that it will be rebuilt every time any unittest is recompiled. But its contents don't actually depend on those tests, it only depends on the names of the files, and that is encoded in the command to run. Since ninja automatically considers the text of the command and rspfile_content as the equivalent of an input (in that if they change, the target is considered dirty and will be rebuilt), these targets don't need any actual inputs.

While you are in there, it would probably be slightly more efficient to use this command since it will spawn many fewer processes: echo "$files" | tr ' ' '\n' > $out.