[ip6] enable multicast loop (#5848)

This commit enables the feature IPV6_MULTICAST_LOOP into OpenThread IPv6
stack. The default-enabled feature allows a node receives a multicast
message sent by its own.

tests/scripts/expect/_common.exp

@@ -125,9 +125,9 @@ proc dispose_all {} {
     }
 }
 
-proc get_mleid {} {
-    send "ipaddr mleid\n"
-    expect "ipaddr mleid"
+proc get_ipaddr {type} {
+    send "ipaddr $type\n"
+    expect "ipaddr $type"
     expect -re {(([0-9a-fA-F]{1,4}:){7,7}[0-9a-fA-F]{1,4})}
     set rval $expect_out(1,string)
     expect "Done"
@@ -144,4 +144,14 @@ proc get_extaddr {} {
     return $rval
 }
 
+proc get_rloc16 {} {
+    send "rloc16\n"
+    expect "rloc16"
+    expect -re {([0-9a-fA-F]{4})}
+    set rval $expect_out(1,string)
+    expect "Done"
+
+    return $rval
+}
+
 set timeout 10