Tests: Exclude bad RunCMake.add_executable case on multi-arch Xcode

The `NoSourcesButLinkObjects` case would not be expected to work under
Xcode with multiple architectures even if the target objects were listed
directly as sources.  Exclude it.  We already exclude similar cases in
`RunCMake.add_library`.

Tests/RunCMake/add_executable/RunCMakeTest.cmake

@@ -2,4 +2,6 @@ include(RunCMake)
 
 run_cmake(NoSources)
 run_cmake(OnlyObjectSources)
-run_cmake(NoSourcesButLinkObjects)
+if(NOT RunCMake_GENERATOR STREQUAL "Xcode" OR NOT "$ENV{CMAKE_OSX_ARCHITECTURES}" MATCHES "[;$]")
+  run_cmake(NoSourcesButLinkObjects)
+endif()