CMake Build: Enable PCH for tests

Because the tests were missing the visibility flags, and PCH targets
that didn't have the visibility flags were skipped, tests were build
without PCH.

Change-Id: I56ddc81fadcb9e1004032a7e6096ea2c0a480a19
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

cmake/QtCreatorAPI.cmake

@@ -681,6 +681,8 @@ function(add_qtc_test name)
   )
 
   set_target_properties(${name} PROPERTIES
+    CXX_VISIBILITY_PRESET hidden
+    VISIBILITY_INLINES_HIDDEN ON
     BUILD_RPATH "${_RPATH_BASE}/${_RPATH}"
     INSTALL_RPATH "${_RPATH_BASE}/${_RPATH}"
   )