add clusters to MutexCheck test, fix silly bug revealed by doing so

The merge lists behavior was flawed in that it would drop one item
from the list per merge, which means that, with high replication
and low number of distinct items, it could even produce an empty
list.

The actual "is there anything wrong" logic is fine, but the list of
clashing values set for a given record is not.

Unfortunately this also doubles the time the test takes, to
21 seconds on MacOS. OW.

api.go

@@ -2692,9 +2692,9 @@ func mergeIDLists(dst []uint64, src []uint64) []uint64 {
 		return dst[i] < dst[j]
 	})
 	// dedup.
-	n := 0
+	n := 1
 	prev := dst[0]
-	for i := 0; i < len(dst); i++ {
+	for i := 1; i < len(dst); i++ {
 		if dst[i] != prev {
 			dst[n] = dst[i]
 			n++
@@ -2712,9 +2712,9 @@ func mergeKeyLists(dst []string, src []string) []string {
 		return dst[i] < dst[j]
 	})
 	// dedup.
-	n := 0
+	n := 1
 	prev := dst[0]
-	for i := 0; i < len(dst); i++ {
+	for i := 1; i < len(dst); i++ {
 		if dst[i] != prev {
 			dst[n] = dst[i]
 			n++

api_test.go

@@ -860,7 +860,13 @@ type mutexCheckField struct {
 }
 
 func TestAPI_MutexCheck(t *testing.T) {
-	c := test.MustRunCluster(t, 3)
+	c := test.MustNewCluster(t, 3)
+	for _, c := range c.Nodes {
+		c.Config.Cluster.ReplicaN = 2
+	}
+	if err := c.Start(); err != nil {
+		t.Fatalf("starting cluster: %v", err)
+	}
 	defer c.Close()
 
 	m0 := c.GetNode(0)