loader: Add a readtest command

readtest will simply load the file in memory, useful for timing
loading on some filesystems.

Reviewed by: tsoome
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D33411

(cherry picked from commit 70661eaafa069062317ae2c6012cc2bc303da0cc)