stackinit: Add union initialization to selftests

The stack initialization selftests were checking scalars, strings,
and structs, but not unions. Add union tests (which are mostly identical
setup to structs). This catches the recent union initialization behavioral
changes seen in GCC 15. Before GCC 15, this new test passes:

    ok 18 test_small_start_old_zero

With GCC 15, it fails:

    not ok 18 test_small_start_old_zero

Specifically, a union with a larger member where a smaller member is
initialized with the older "= { 0 }" syntax:

union test_small_start {
     char one:1;
     char two;
     short three;
     unsigned long four;
     struct big_struct {
             unsigned long array[8];
     } big;
};

This is a regression in compiler behavior that Linux has depended on.
GCC does not seem likely to fix it, instead suggesting that affected
projects start using -fzero-init-padding-bits=unions:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118403

Link: https://lore.kernel.org/r/20250127191031.245214-2-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>

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