Cluster regression test for #3043.

The test works but is very slow so far, since it involves resharding
1/5 of all the cluster slots from master 0 to the other 4 masters and
back into the original master.

tests/cluster/tests/12-replica-migration-2.tcl

+# Replica migration test #2.
+#
+# Check that the status of master that can be targeted by replica migration
+# is acquired again, after being getting slots again, in a cluster where the
+# other masters have slaves.
+
+source "../tests/includes/init-tests.tcl"
+
+# Create a cluster with 5 master and 10 slaves, so that we have 2
+# slaves for each master.
+test "Create a 5 nodes cluster" {
+    create_cluster 5 10
+}
+
+test "Cluster is up" {
+    assert_cluster_state ok
+}
+
+test "Each master should have two replicas attached" {
+    foreach_redis_id id {
+        if {$id < 5} {
+            wait_for_condition 1000 50 {
+                [llength [lindex [R 0 role] 2]] == 2
+            } else {
+                fail "Master #$id does not have 2 slaves as expected"
+            }
+        }
+    }
+}
+
+set master0_id [dict get [get_myself 0] id]
+test "Resharding all the master #0 slots away from it" {
+    set output [exec \
+        ../../../src/redis-trib.rb rebalance \
+        --weight ${master0_id}=0 \
+        127.0.0.1:[get_instance_attrib redis 0 port] >@ stdout]
+}
+
+test "Master #0 should lose its replicas" {
+    wait_for_condition 1000 50 {
+        [llength [lindex [R 0 role] 2]] == 0
+    } else {
+        fail "Master #0 still has replicas"
+    }
+}
+
+test "Resharding all the master #0 slots back to it" {
+    set output [exec \
+        ../../../src/redis-trib.rb rebalance \
+        --weight ${master0_id}=1 \
+        --use-empty-masters \
+        127.0.0.1:[get_instance_attrib redis 0 port]]
+}
+
+test "Master #0 should re-acquire one or more replicas" {
+    wait_for_condition 1000 50 {
+        [llength [lindex [R 0 role] 2]] >= 1
+    } else {
+        fail "Master #0 has no has replicas"
+    }
+}