mod: upgraded a couple dependencies

dragonboat: bumped version to v3.3.7

dragonboat: fixed #260

dragonboat: bumped version number to v3.3.5

dragonboat: bumped version to v3.3.4

mod: added github.com/stretchr/testify

mod: bumped pebble version

rsm: not to call ManagedStateMachine.Prepare() when saving dummy snap…

…shots

Fixes #162

Merge branch 'master' into release-3.3

dragonboat: bumped version number to v3.2.8