Add a placeholder for the empty vm directory

Without it, the clone/checkout does not create the vm directory
Allow subdirectories in test/ directory
remove debugging messages
LoadReg (and other special registers) now initialized to 0 when creating new user thread
fix compilation error (unused variable) when not in user mode
Merge branch 'master' of https://forge.imag.fr/anonscm/git/system/CSE/nachos
Suppression des test

Les tests ne compilent plus avec les motifs du système de build