test: assert cluster.disconnect is async

See joyent/node#8043, test passed on v0.11 already, but this makes the
test stronger.

Reviewed-by: Trevor Norris <trev.norris@gmail.com>
path: fix slice OOB in trim

Internal function trim(arr). 2nd parameter of slice() should be slice's
end index (not included). Because of function normalize() (called before
trim()), "start" is always zero so the bug -for now- has no effect, but
its a bug waiting to happen.

Reviewed-by: Trevor Norris <trev.norris@gmail.com>
test: disable 0-dgram tests on darwin

fix #8023
Merge remote-tracking branch 'upstream/master'