test/cast_sizet_to_uint32: add int32_t (signed) cast case

Exercises the signed branch of the new SizeT \xE2\x86\x92 Int cast handling.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: KimayaBedarkar

test/cast_sizet_to_uint32/cast_sizet_to_uint32.c

@@ -8,3 +8,10 @@ uint32_t sizet_to_u32(size_t n)
 {
   return (uint32_t) n;
 }
+
+int32_t sizet_to_i32(size_t n)
+  _requires((_specint) n <= INT32_MAX)
+  _ensures((_specint) return == n)
+{
+  return (int32_t) n;
+}