test: fix flaky TestAutopilot_CleanupDeadServer

The fix seems to be related to the pointer comparison and swapping we
did around killing a non-leader. I actually can't quite explain it, but
when comparing against Consul's version of this test I noticed they used
the slice index to track the killed server instead of pointer swapping.

As soon as I switched to slice index tracking I could no longer
reproduce the failure.

In addition:
- Tested membership counts on all servers instead of just 1 for added
  correctness.
- Stopped testing raft v1 because it is unsupported.

nomad/autopilot_test.go

@@ -68,11 +68,8 @@ func wantRaft(servers []*Server) error {
 
 func TestAutopilot_CleanupDeadServer(t *testing.T) {
 	t.Parallel()
-	for i := 1; i <= 3; i++ {
-		t.Run(fmt.Sprintf("raft version: %v", i), func(t *testing.T) {
-			testCleanupDeadServer(t, i)
-		})
-	}
+	t.Run("raft_v2", func(t *testing.T) { testCleanupDeadServer(t, 2) })
+	t.Run("raft_v3", func(t *testing.T) { testCleanupDeadServer(t, 3) })
 }
 
 func testCleanupDeadServer(t *testing.T, raftVersion int) {
@@ -93,9 +90,10 @@ func testCleanupDeadServer(t *testing.T, raftVersion int) {
 	servers := []*Server{s1, s2, s3}
 
 	// Try to join
-	TestJoin(t, s1, s2, s3)
+	TestJoin(t, servers...)
 
 	for _, s := range servers {
+		testutil.WaitForLeader(t, s.RPC)
 		retry.Run(t, func(r *retry.R) { r.Check(wantPeers(s, 3)) })
 	}
 
@@ -104,26 +102,37 @@ func testCleanupDeadServer(t *testing.T, raftVersion int) {
 	defer cleanupS4()
 
 	// Kill a non-leader server
-	if leader := waitForStableLeadership(t, servers); leader == s3 {
-		s3, s1 = s1, s3
+	killedIdx := 0
+	for i, s := range servers {
+		if !s.IsLeader() {
+			killedIdx = i
+			s.Shutdown()
+			break
+		}
 	}
-	s3.Shutdown()
 
 	retry.Run(t, func(r *retry.R) {
-		alive := 0
-		for _, m := range s1.Members() {
-			if m.Status == serf.StatusAlive {
-				alive++
+		for i, s := range servers {
+			alive := 0
+			if i == killedIdx {
+				// Skip shutdown server
+				continue
+			}
+			for _, m := range s.Members() {
+				if m.Status == serf.StatusAlive {
+					alive++
+				}
+			}
+
+			if alive != 2 {
+				r.Fatalf("expected 2 alive servers but found %v", alive)
 			}
-		}
-		if alive != 2 {
-			r.Fatalf("expected 2 alive servers but found %v", alive)
 		}
 	})
 
 	// Join the new server
-	TestJoin(t, s1, s2, s4)
-	servers[2] = s4
+	servers[killedIdx] = s4
+	TestJoin(t, servers...)
 
 	waitForStableLeadership(t, servers)