raft: test: do not check snapshot during backpressure test
raft: test: do not check snapshot during backpressure test

Unfortunately snapshot checking still does not work in the presence of
log entries reordering. It is impossible to know when exactly the
snapshot will be taken and if it is taken before all smaller than
snapshot idx entries are applied the check will fail since it assumes
that.

This patch disabled snapshot checking for SUM state machine that is used
in backpressure test.

Message-Id: <26526a4985d693c9bf78e30745c1a153dd30afad@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla