[test] include CLI tests in RCP mode (#5159)

This makes expect run all cli-*.exp and posix-*.exp tests in RCP mode.
New scripts should use the function spawn_node in _common.exp to spawn
new processes. This is compatible with both simulation mode and RCP
mode.

tests/scripts/expect/cli-counters.exp

@@ -27,11 +27,10 @@
 #  POSSIBILITY OF SUCH DAMAGE.
 #
 
-spawn $env(OT_COMMAND) 1
+source "tests/scripts/expect/_common.exp"
+
+set spawn_id [spawn_node 1]
 set timeout 1
-expect_after {
-    timeout { exit 1 }
-}
 
 send "counters\n"
 expect "mac"
@@ -51,3 +50,5 @@ send "counters mle 1\n"
 expect "Error 7: InvalidArgs"
 send "counters other\n"
 expect "Error 7: InvalidArgs"
+
+dispose