Fix one of flakey test in protocol tests

Author: ono-max

test/support/protocol_test_case.rb

@@ -175,12 +175,16 @@ def req_step_back
     end
 
     def req_terminate_debuggee
+      # Kernel#exit method will be called when the debuggee receives the following method.
       case get_target_ui
       when 'vscode'
         send_dap_request 'terminate'
       when 'chrome'
         send_cdp_request 'Runtime.terminateExecution'
       end
+      # Ensure debuggee exit by waiting for Kernel#exit in server_cdp.rb to complete.
+      # This `wait_pid` is called to prevent disconnection before the debuggee exits.
+      flunk create_protocol_msg test_info, "Expected the debuggee program to finish" unless wait_pid @remote_info.pid, 3
 
       close_reader
     end