Reported upstream: https://bugreports.qt.io/browse/PYSIDE-1430

--- a/sources/pyside2/tests/CMakeLists.txt	2020-11-11 13:51:30.000000000 +0100
+++ b/sources/pyside2/tests/CMakeLists.txt	2020-11-24 11:42:34.629110123 +0100
@@ -7,8 +7,7 @@
 
     # BUILD_DIR and QT_DIR are used by init_paths.py for setting
     # the path to the testbinding module
-    get_filename_component(BUILD_DIR "${CMAKE_BINARY_DIR}" DIRECTORY)
-    get_filename_component(BUILD_DIR "${CMAKE_BINARY_DIR}" DIRECTORY)
+    set(BUILD_DIR "${CMAKE_BINARY_DIR}")
     set(QT_DIR "${_qt5Core_install_prefix}")
 
     macro(TEST_QT_MODULE var name)