Add dummy CLUSTER SLAVES call tests to fix reply ci (redis#12224)

In redis#12166, we removed a call to CLUSTER SLAVES, which
then caused reply-schemas ci to fail:
```
WARNING! The following commands were not hit at all:
  cluster|slaves
  ERROR! at least one command was not hit by the tests
```

Because we already have command output that cover CLUSTER REPLICAS
elsewhere, here we simply add some dummy tests to fix the ci.

tests/cluster/tests/00-base.tcl

@@ -82,3 +82,8 @@ test "Coverage: CLUSTER HELP" {
 test "Coverage: ASKING" {
     assert_equal {OK} [R 0 ASKING]
 }
+
+test "CLUSTER SLAVES and CLUSTER REPLICAS with zero replicas" {
+    assert_equal {} [R 0 cluster slaves [R 0 CLUSTER MYID]]
+    assert_equal {} [R 0 cluster replicas [R 0 CLUSTER MYID]]
+}

tests/cluster/tests/05-slave-selection.tcl

@@ -16,11 +16,20 @@ test "Cluster is up" {
 test "The first master has actually two slaves" {
     wait_for_condition 1000 50 {
         [llength [lindex [R 0 role] 2]] == 2
+        && [llength [R 0 cluster replicas [R 0 CLUSTER MYID]]] == 2
     } else {
         fail "replicas didn't connect"
     }
 }
 
+test "CLUSTER SLAVES and CLUSTER REPLICAS output is consistent" {
+    # Because we already have command output that cover CLUSTER REPLICAS elsewhere,
+    # here we simply judge whether their output is consistent to cover CLUSTER SLAVES.
+    set res [R 0 cluster slaves [R 0 CLUSTER MYID]]
+    set res2 [R 0 cluster replicas [R 0 CLUSTER MYID]]
+    assert_equal $res $res2
+}
+
 test {Slaves of #0 are instance #5 and #10 as expected} {
     set port0 [get_instance_attrib redis 0 port]
     assert {[lindex [R 5 role] 2] == $port0}