[expect] refactor common functions (#5634)

* unify node creating and destroying
* simplify switching between nodes

tests/scripts/expect/_multinode.exp

@@ -29,27 +29,21 @@
 
 source "tests/scripts/expect/_common.exp"
 
-proc setup_nodes {{childmode {r}}} {
+proc setup_two_nodes {{childmode {r}}} {
     # Sets up a Thread network with 2 nodes, spawn_1 as the leader and spawn_2
     # as a child.
-
-    global spawn_1
-    global spawn_2
-    global spawn_id
-    global env
-
-    set spawn_2 [spawn_node 2]
     set psk "J01NME"
 
-    set spawn_id $spawn_2
+    spawn_node 2
     send "eui64\n"
     expect -re {([0-9a-f]{16})}
     set eui64 $expect_out(1,string)
     expect "Done"
 
+    # Sets up a Thread network with node 1 as the leader.
+    spawn_node 1
     setup_leader
 
-    set spawn_id $spawn_1
     send "commissioner start\n"
     expect "Done"
     expect "Commissioner: active"
@@ -61,7 +55,7 @@ proc setup_nodes {{childmode {r}}} {
     set channel $expect_out(1,string)
     expect "Done"
 
-    set spawn_id $spawn_2
+    switch_node 2
     send "mode $childmode\n"
     expect "Done"
     send "ifconfig up\n"
@@ -74,12 +68,3 @@ proc setup_nodes {{childmode {r}}} {
     wait_for "state" "child"
     expect "Done"
 }
-
-proc dispose_nodes {} {
-    global spawn_2
-    global spawn_id
-
-    dispose_leader
-    set spawn_id $spawn_2
-    dispose
-}