CMake: Don't recurse with qt-cmake-standalone-test in-source dirs

When the qt-cmake-standalone-test root project is configured in-source
in the source directory of a test A, where test A ends up being added
via add_subdirectory(A), this causes the SUBDIRECTORIES property of A
to contain itself for some reason.
Recursively iterating over the root project's subdirectories thus
causes an endless recursion.

Make sure to remove the currently processed directory from the
retrieved SUBDIRECTORIES property when the function is called
during qt-cmake-standalone-test configuration.

We limit it just for that script, so that we don't accidentally
increase the configure time when building all standalone tests, and
call list(REMOVE_ITEM) too many times.

Upstream issue filed at
https://gitlab.kitware.com/cmake/cmake/-/issues/25489

In a way, amends d08fa86e63 because it
exposed the issue.
Amends 1c82e92202

Pick-to: 6.5 6.6 6.7
Task-number: QTBUG-119998
Change-Id: I6f92a07be105cde74ac4946523967791db7bf301
Reviewed-by: Amir Masoud Abdol <amir.abdol@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

cmake/QtPublicCMakeHelpers.cmake

@@ -117,6 +117,18 @@ function(_qt_internal_collect_buildsystem_targets result dir)
     endif()
 
     get_property(subdirs DIRECTORY "${dir}" PROPERTY SUBDIRECTORIES)
+
+    # Make sure that we don't hit endless recursion when running qt-cmake-standalone-test on a
+    # in-source test dir, where the currently processed directory lists itself in its SUBDIRECTORIES
+    # property.
+    # See https://bugreports.qt.io/browse/QTBUG-119998
+    # and https://gitlab.kitware.com/cmake/cmake/-/issues/25489
+    # Do it only when QT_INTERNAL_IS_STANDALONE_TEST is set, to avoid the possible slowdown when
+    # processing many subdirectores when configuring all standalone tests rather than just one.
+    if(QT_INTERNAL_IS_STANDALONE_TEST)
+        list(REMOVE_ITEM subdirs "${dir}")
+    endif()
+
     foreach(subdir IN LISTS subdirs)
         _qt_internal_collect_buildsystem_targets(${result} "${subdir}" ${forward_args})
     endforeach()