Turn core:math/bìg tests into regular core:testing tests.

`core:math/big` has been verified against Python's big integer implementation long enough.
Turn it into a regular regression test using the `core:testing` framework, testing against
a generated corpus of test vectors.

core/math/big/radix.odin

@@ -310,7 +310,7 @@ int_atoi :: proc(res: ^Int, input: string, radix := i8(10), allocator := context
 		res.sign = sign
 	}
 
-	return nil
+	return internal_clamp(res)
 }