kunit/fortify: Expand testing of __compiletime_strlen()

It seems that Clang thinks __builtin_constant_p() of undefined variables
should return true[1]. This is being fixed separately[2], but in the
meantime, expand the fortify tests to help track this kind of thing down
faster in the future.

Link: https://github.com/ClangBuiltLinux/linux/issues/2073 [1]
Link: https://github.com/llvm/llvm-project/pull/130713 [2]
Link: https://lore.kernel.org/r/20250312000349.work.786-kees@kernel.org


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