geli: optimize tests

Reduce the geli tests' runtime by about a third:

• In integrity_test:copy, use a file-backed md(4) device instead of a malloc'd one. That way we can corrupt the underlying storage without needing to detach and reattach the geli device.

• In integrity_test:{copy, hmac, data} and onetime_test:{onetime, onetime_a}, move reads of /dev/random out of the loop.

MFC after: 2 weeks