Use DL_PATHS again for correct test discovery env

Not sure why but this has become necessary again for running on Ubuntu24
in an LCG/dev3 environment to run test discovery succesfully. Otherwise
the environment for discovery is not set properly and running the
executable (during the build stage) fails with a symbol lookup error.
The latter is caused by the fact that, LCGs podio is not built with SIO
support.

tests/unittests/CMakeLists.txt

@@ -78,6 +78,7 @@ else()
       WORKING_DIRECTORY ${CMAKE_CURRENT_LIST_DIR}
       TEST_PREFIX "UT_" # make it possible to filter easily with -R ^UT
       TEST_SPEC ${filter_tests} # discover only tests that are known to not fail
+      DL_PATHS ${CMAKE_CURRENT_BINARY_DIR}:${PROJECT_BINARY_DIR}/src:${PROJECT_BINARY_DIR}/tests:$<TARGET_FILE_DIR:ROOT::Tree>:$<$<TARGET_EXISTS:SIO::sio>:$<TARGET_FILE_DIR:SIO::sio>>:$ENV{LD_LIBRARY_PATH}
       PROPERTIES
         ENVIRONMENT
         PODIO_SIOBLOCK_PATH=${PROJECT_BINARY_DIR}/tests