[cli] disallow concurrent commands (#6695)

This commit disallows concurrent commands.

With this commit, the CLI only execute a new command after it complete
the previous command. CLI can also prompt properly after the command
execution is done.

Other fixes and enhancements:
- Fixes premature command prompt
- Add ping async command for ping in async mode: output Done
  immediately but print ping responses later on.
- Fixes networkdiagnostic get outputs multiple Done by always waiting
  for 5 seconds.

tests/scripts/expect/_common.exp

@@ -58,6 +58,7 @@ proc wait_for {command success {failure {[\r\n]FAILURE_NOT_EXPECTED[\r\n]}}} {
 }
 
 proc expect_line {line} {
+    set timeout 10
     expect -re "\[\r\n \]($line)(?=\[\r\n>\])"
     return $expect_out(1,string)
 }