The failure happened in the cleanup phase of the test case, in which ATF attempts to recursively remove the temporary work directory. When it attempted to remove one of the directories inside it, it failed with a ENOENT message, which in SunOS may mean that the directory is not empty. Strangely, when inspecting the left-over work tree, that directory was indeed empty and it could not be removed with rm -rf nor with rmdir.
The manual page for unlink(2) finally gave me the clue of what was happening:
If the path argument is a directory and the filesystem supports unlink() and unlinkat() on directories, the directory is unlinked from its parent with no cleanup being performed. In UFS, the disconnected directory will be found the next time the filesystem is checked with fsck(1M). The unlink() and unlinkat() functions will not fail simply because a directory is not empty. The user with appropriate privileges can orphan a non-empty directory without generating an error message.The solution was easy: as my custom remove function is supposed to remove files only, I added a check before the call to unlink(2) to ensure that the path name does not point to a directory. Not the prettiest possibility (because it is subject to race-conditions even though it is not critical), but it works.