Add CLIs support TWAMP Light feature according to sonic-net/SONiC HLD…

… PR#1320

Signed-off-by: Xianghong Gu <[email protected]>

tests/twamp_light_test.py

@@ -115,13 +115,13 @@
     """Usage: add [OPTIONS] <session_name> <sender_ip:port> <reflector_ip:port>
            <packet_count> <tx_interval> <timeout> <statistics_interval>
 
-Error: Invalid value for "<session_name>": """ + TL_SESSION_NAME + """. TWAMP-Light session already exist
+Error: Invalid value for "<session_name>": """ + TL_SESSION_NAME + """. TWAMP-Light session already exists
 """
 
 twamp_light_start_session_without_session_output = \
     """Usage: start [OPTIONS] <session_name|all>
 
-Error: Invalid value for "<session_name>": """ + TL_SESSION_NAME + """. TWAMP-Light session not exist
+Error: Invalid value for "<session_name>": """ + TL_SESSION_NAME + """. TWAMP-Light session does not exist
 """
 
 twamp_light_add_continous_sender_with_invalid_monitor_time_output = \