osd/OSDCap: add unit test for parsing pools/objects with _ and -

Hunting #4122, where a user saw

2013-02-13 19:39:25.467916 7f766fdb4700 10 osd.0 10  session 0x2c8cc60 client.libvirt has caps osdcap[grant(object_prefix rbd^@children  class-read),grant(pool libvirt^@pool^@test rwx)] 'allow class-read object_prefix rbd_children, allow pool libvirt-pool-test rwx'

Signed-off-by: Sage Weil <sage@inktank.com>