lpf_test: correct threshold ranges

the random number generator creates values from [0, range) add 1 to all
and make hev more realistic by mirroring its calculation of level >> 4,
i.e., [0, 3]

cherry-picked from libvpx:
5203b40a2 lpf_test: correct threshold ranges

Change-Id: I20508cce088386b41b18418b60a94d34b2c88e1e