Fix stringop-overflow warning in test-strncat.

Starting with commit
b2c474f8
"x86: Fix strncat-avx2.S reading past length [BZ #30065]"

Building on s390 the test fails due warnings like:

In function ‘do_one_test’,
    inlined from ‘do_overflow_tests’ at test-strncat.c:175:7:
test-strncat.c:31:18: error: ‘strnlen’ specified bound [4294966546, 4294967295] exceeds maximum object size 2147483647 [-Werror=stringop-overflow=]
   31 | # define STRNLEN strnlen
      |                  ^
test-strncat.c:83:16: note: in expansion of macro ‘STRNLEN’
   83 |   size_t len = STRNLEN (src, n);
|                ^~~~~~~

In all werror cases, the call to strnlen (.., SIZE_MAX) is inlined.
Therefore this patch just marks the do_one_test function as noinline.

Reviewed-by: Wilco Dijkstra <Wilco.Dijkstra@arm.com>