Autogen: Fix support for multiple targets sharing MOCed sources in Xcode "new build system"
In !5229 (merged) I need to suppress part of the
Tests/QtAutogen/MocSkipSource/CMakeLists.txt test under the Xcode new build system. Otherwise, it fails because the test case adds the same source to multiple independent targets, and mocs it in both. That causes the generated custom command to appear multiple times in separate targets, which is not allowed by the Xcode new build system.