Fix flaky raft tests, checking for candidate state

[dhiaayachi]

In some tests in raft_test.go we have the following pattern:

// wait for the remaining follower to trigger an election
waitForState(follower, Candidate)
require.Equal(t, Candidate, follower.getState())

The issue with this pattern is that candidate state is a transient state and a node is not expected to stay in it for a long time, so by the time we finish waitForState(follower, Candidate) and get the state again the new node state could already have changed.

[jrasell]

LGTM!