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/internal.odin

@@ -1660,13 +1660,13 @@ internal_int_sqrt :: proc(dest, src: ^Int, allocator := context.allocator) -> (e
 
 		if internal_gte(y, x) {
 			internal_swap(dest, x)
-			return nil
+			return internal_clamp(dest)
 		}
 		internal_swap(x, y)
 	}
 
 	internal_swap(dest, x)
-	return err
+	return internal_clamp(dest)
 }
 internal_sqrt :: proc { internal_int_sqrt, }
 

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)
 }