Generate rollback delta in a test subdirectory

Signed-off-by: Frédéric Dalleau <frederic.dalleau@collabora.com>

Jenkinsfile

@@ -266,12 +266,13 @@ def buildOStreeRollbackDelta(architecture, type, board, debosarguments = "", rep
         --tree=ref=${branch} \
         --branch=${rollback_branch} \
         --skip-list="skip_list"
+      mkdir -p tests
       ostree --repo=${repo} static-delta generate \
         --from=${branch} \
         --to=${rollback_branch} \
         --inline \
         --min-fallback-size=1024 \
-        --filename ${image_name}_rollback.delta""")
+        --filename tests/${image_name}_rollback.delta""")
 }
 
 /** Generate the image name