Fix another cmake error

   CMake Error at CMakeLists.txt:229 (add_custom_command):
    Exactly one of PRE_BUILD, PRE_LINK, or POST_BUILD must be given.

Author: christophfroehlich

CMakeLists.txt

@@ -227,6 +227,7 @@ if(BUILD_TESTING)
     set(_resource "$<TARGET_FILE:${test_plugin_target}>")
     set(_output "${test_prefix}/lib/$<TARGET_FILE_NAME:${test_plugin_target}>")
     add_custom_command(TARGET ${test_plugin_target}
+      POST_BUILD
       COMMENT "Copying target ${test_plugin_target} for testing"
       COMMAND ${CMAKE_COMMAND} -E copy_if_different
         "${_resource}"