Fix stringop-overflow warning in tst-strlcat2.

On s390x, I get warnings like this when do_one_test is inlined with SIZE_MAX:
In function ‘do_one_test’,
    inlined from ‘do_overflow_tests’ at tst-strlcat2.c:184:2:
tst-strlcat2.c:49:18: error: ‘strnlen’ specified bound [18446744073709550866, 18446744073709551615] exceeds maximum object size 9223372036854775807 [-Werror=stringop-overflow=]
   49 | # define STRNLEN strnlen
      |                  ^
tst-strlcat2.c:89:23: note: in expansion of macro ‘STRNLEN’
   89 |   size_t dst_length = STRNLEN (dst, n);
      |                       ^~~~~~~

This patch just marks the do_one_test function as noinline as also done in test-strncat.c:
Fix stringop-overflow warning in test-strncat.
https://sourceware.org/git/?p=glibc.git;a=commit;h=51aeab9a363a0d000d0912aa3d6490463a26fba2