kunit/overflow: Add tests for STACK_FLEX_ARRAY_SIZE() helper

Add a couple of tests for new STACK_FLEX_ARRAY_SIZE() helper.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/c127631a03cdd7f59bfa091b9666a93bf69d0322.1745355442.git.gustavoars@kernel.org


Signed-off-by: Kees Cook <kees@kernel.org>