[github-actions] fix expects fail by chance (#5157)

This commit tries to reduce expects fails in GitHub actions:
- wait_for only sends command when it's not empty
- increases the default expect timeout from 1s to 3s.

tests/scripts/expect/_common.exp

@@ -30,7 +30,10 @@
 proc wait_for {command expected} {
     set result 0
     for {set i 0} {$i < 20} {incr i} {
-        send "$command\n"
+        if {$command != ""} {
+            send "$command\n"
+        }
+
         expect {
             -re $expected {
                 set result 1