Refactor RaftActorTest to avoid test failures

Refactor RaftActorTest to avoid test failures

The "doReturn(value).when(mockObj).coolMethod(same(arg))" way of mocking
the return values for "coolMethod()" calls resulted in
UnfinishedStubbingException, even though there seems to be no reason
for such error. This change introduces "the other" way of such mocking:
"when(mockObj.coolMethod(same(arg)).thenReturn()".

JIRA: CONTROLLER-2080
Change-Id: I58b9c08780c27b43ad5eff4e21c921b9260fc236
Signed-off-by: Šimon Ukuš <simon.ukus@pantheon.tech>
Signed-off-by: Samuel Schneider <samuel.schneider@pantheon.tech>