Swift: Update test case to try CMP0126 NEW behavior

Extend the `SwiftOnly` test to cover the fix in commit 3ddd7f3576
(enable_language: Fix test for working compiler with CMP0126 NEW
behavior, 2021-07-15), as that commit did for the `CSharpOnly` test.

Fixes: #22451

Tests/SwiftOnly/CMakeLists.txt

@@ -1,4 +1,7 @@
 cmake_minimum_required(VERSION 3.3)
+if(POLICY CMP0126)
+  cmake_policy(SET CMP0126 NEW)
+endif()
 
 # NOTE: Force the Release mode configuration as there are some issues with the
 # debug information handling on macOS on certain Xcode builds.