tests: fix build against upcoming `gcc-14` (`-Werror=calloc-transposed-args`)

tests: fix build against upcoming `gcc-14` (`-Werror=calloc-transposed-args`)

`gcc-14` added a new `-Wcalloc-transposed-args` warning recently. It
detected minor infelicity in `calloc()` API usage in `elfutils`:

    elfstrmerge.c: In function 'main':
    elfstrmerge.c:450:32: error:
      'calloc' sizes specified with 'sizeof' in the earlier argument and not in the later argument [-Werror=calloc-transposed-args]
      450 |   newscnbufs = calloc (sizeof (void *), newshnums);
          |                                ^~~~
    elfstrmerge.c:450:32: note: earlier argument should specify number of elements, later size of each element

Signed-off-by: Sergei Trofimovich <slyich@gmail.com>