cmake: add swift test

cmake/test/swift/module.modulemap

@@ -0,0 +1,4 @@
+module SDL3 [extern_c] {
+    header "shim.h"
+    export *
+}

cmake/test/swift/shim.h

@@ -0,0 +1,3 @@
+/* Contributed by Piotr Usewicz (https://github.com/pusewicz) */
+
+#include <SDL3/SDL.h>