Tests: Fix RunCMake.CXXModules compilation with Clang 21

Clang 21 now enforces placement of the module declaration:

    .../scan_properties/module.cxx:7:8: error: module
       declaration must occur at the start of the translation unit

Tests/RunCMake/CXXModules/examples/scan_properties/module.cxx

@@ -1,11 +1,11 @@
+export module M;
+
 #if SCANNING_CONTROL
 #  ifndef CMAKE_SCANNED_THIS_SOURCE
 #    error "This file should have been scanned"
 #  endif
 #endif
 
-export module M;
-
 export int from_module()
 {
   return 0;