We sometimes encounter this issue during the init of REST tests.

tests-1    | Install graphontrackersv5...[OK]
tests-1    | 
tests-1    | In delete_directory.php line 62:
tests-1    |                                                                                
tests-1    |   Failed to delete directory "/var/tmp/tuleap_cache/valinor_cache/": rmdir(/v  
tests-1    |   ar/tmp/tuleap_cache/valinor_cache/): Directory not empty.                    
tests-1    |                                                                                
tests-1    | 
tests-1    | plugin:install [--all] [--] [<plugins>...]
tests-1    | 
tests-1    | Install frs...