tests: Add read-only fsop benchmark

Useful for getting a notion of response time and throughput
on sequential read operations.
Random read option should be added later on.
Currently being used by me to measure read performance on
compressed vs uncompressed data.

Example output:
OSv v0.08-160-gddb9322
eth0: 192.168.122.15
/zpool.so: 96kb: 1.77ms, (+1.77ms)
/libzfs.so: 211kb: 6.57ms, (+4.80ms)
/zfs.so: 96kb: 8.25ms, (+1.68ms)
/tools/mkfs.so: 10kb: 9.32ms, (+1.07ms)
/tools/cpiod.so: 244kb: 14.08ms, (+4.76ms)
...
/usr/lib/jvm/jre/lib/content-types.properties: 5kb: 1066.17ms, (+2.87ms)
/usr/lib/jvm/jre/lib/cmm/GRAY.pf: 556b: 1066.74ms, (+0.57ms)
/usr/lib/jvm/jre/lib/cmm/CIEXYZ.pf: 784b: 1067.34ms, (+0.60ms)
/usr/lib/jvm/jre/lib/cmm/sRGB.pf: 6kb: 1067.96ms, (+0.62ms)
/usr/lib/jvm/jre/lib/cmm/LINEAR_RGB.pf: 488b: 1068.61ms, (+0.64ms)
/usr/lib/jvm/jre/lib/cmm/PYCC.pf: 228kb: 1073.96ms, (+5.36ms)
/usr/lib/jvm/jre/lib/sound.properties: 1kb: 1074.65ms, (+0.69ms)

REPORT
-----
Files:	552
Read:	127395kb
Time:	1074.65ms
MBps:	115.39

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>