[test] make test_diag less flaky (#11008)

This commit expects echo of diag commands to make sure the command is
received by the node when waiting for the expected output. This helps
make this test less flaky.

tests/scripts/thread-cert/node.py

@@ -936,7 +936,7 @@ class NodeImpl:
         cmd = cmd.strip()
         while True:
             line = self.__readline()
-            if line == cmd:
+            if line.strip() == cmd:
                 break
 
             logging.warning("expecting echo %r, but read %r", cmd, line)