[devtools] fix a couple issues with xvfb-run
xvfb-run didn't quite clean up correctly after itself, because:
- dash doesn't run EXIT traps on signals
- if we don't run the command in the background, a TERM handler doesn't run
until the command exits
# type attribute might already be there if migrating from
# version < 3.5 to version >= 3.5.3, BaseTransition being added
# in bootstrap_migration
if versions_map['cubicweb'][0] >= (3, 5, 0):
add_attribute('BaseTransition', 'type')
sync_schema_props_perms('state_of')
sync_schema_props_perms('transition_of')