Disable -Waggressive-loop-optimizations warnings in tst-dynarray.c

Disable -Waggressive-loop-optimizations warnings in tst-dynarray.c

My build-many-glibcs.py bot shows -Waggressive-loop-optimizations
errors building the glibc testsuite for 32-bit architectures with GCC
mainline, which seem to have appeared between GCC commits
4abc0c196b10251dc80d0743ba9e8ab3e56c61ed and
d8edfadfc7a9795b65177a50ce44fd348858e844:

In function 'dynarray_long_noscratch_resize',
    inlined from 'test_long_overflow' at tst-dynarray.c:489:5,
    inlined from 'do_test' at tst-dynarray.c:571:3:
../malloc/dynarray-skeleton.c:391:36: error: iteration 1073741823 invokes undefined behavior [-Werror=aggressive-loop-optimizations]
  391 |             DYNARRAY_ELEMENT_INIT (&list->u.dynarray_header.array[i]);
tst-dynarray.c:39:37: note: in definition of macro 'DYNARRAY_ELEMENT_INIT'
   39 | #define DYNARRAY_ELEMENT_INIT(e) (*(e) = 23)
      |                                     ^
In file included from tst-dynarray.c:42:
../malloc/dynarray-skeleton.c:389:37: note: within this loop
  389 |         for (size_t i = old_size; i < size; ++i)
      |                                   ~~^~~~~~
In function 'dynarray_long_resize',
    inlined from 'test_long_overflow' at tst-dynarray.c:479:5,
    inlined from 'do_test' at tst-dynarray.c:571:3:
../malloc/dynarray-skeleton.c:391:36: error: iteration 1073741823 invokes undefined behavior [-Werror=aggressive-loop-optimizations]
  391 |             DYNARRAY_ELEMENT_INIT (&list->u.dynarray_header.array[i]);
tst-dynarray.c:27:37: note: in definition of macro 'DYNARRAY_ELEMENT_INIT'
   27 | #define DYNARRAY_ELEMENT_INIT(e) (*(e) = 17)
      |                                     ^
In file included from tst-dynarray.c:28:
../malloc/dynarray-skeleton.c:389:37: note: within this loop
  389 |         for (size_t i = old_size; i < size; ++i)
      |                                   ~~^~~~~~

I don't know what GCC change made these errors appear, or why they
only appear for 32-bit architectures.  However, the warnings appear to
be both true (that iteration would indeed involve undefined behavior
if executed) and useless in this particular case (that iteration is
never executed, because the allocation size overflows and so the
allocation fails - but the check for allocation size overflow is in a
separate source file and so can't be seen by the compiler when
compiling this test).  So use the DIAG_* macros to disable
-Waggressive-loop-optimizations around the calls in question to
dynarray_long_resize and dynarray_long_noscratch_resize in this test.

Tested with build-many-glibcs.py (GCC mainline) for arm-linux-gnueabi,
where it restores a clean testsuite build.