Either elf/tst-dlopenrpath.c is bogus or open_path in elf/dl-load.c is wrong.
open_path has

/* Try to open NAME in one of the directories in *DIRSP.
   Return the fd, or -1.  If successful, fill in *REALNAME
   with the malloc'd full directory name.  If it turns out
   that none of the directories in *DIRSP exists, *DIRSP is
   replaced with (void *) -1, and the old value is free()d
   if MAY_FREE_DIRS is true.  */
                                                                                
static int
open_path (const char *name, size_t namelen, int preloaded,
           struct r_search_path_struct *sps, char **realname,
           struct filebuf *fbp)

So when a DSO has a DT_RPATH which isn't used to locate a DT_NEEDED
entry, that DT_RPATH may be ignored for this DSO later on for any dlopen
calls made from the DSO. elf/tst-dlopenrpathmod.c has

xxxxx          (RPATH)              Library rpath: [$ORIGIN/test-subdir]

If DT_RPATH is searched when loading tst-dlopenrpathmod.so for some reason,
DT_RPATH will be ignored for dlopen calls in tst-dlopenrpathmod.so. I got

couldn't open in-subdir.so from foo