raft: add tests for maybeappend

add this one?
```
{
lastterm - 3, lastindex - 3, lastindex, []pb.Entry{},
1, true, 1, false, // committed index cannot roll back
}
{
lastterm - 3, lastindex - 3, lastindex - 3, []pb.Entry{{Term: 1}, {Term: 2}},
1, true, 1, false, // committed index cannot roll back
}
```