Update timeout for gnmi subscribe API (sonic-net#19019)

What is the motivation for this PR?
test_gnmi_configdb_streaming_onchange_01 failed on KVM DUT, and test logs show that it needs more than 30 seconds to get all the results from GNMI server.

How did you do it?
Update timeout from 30 seconds to 120 seconds.

How did you verify/test it?
Run gnmi end to end test.

Author: ganglyu

tests/gnmi/helper.py

@@ -431,7 +431,7 @@ def gnmi_subscribe_streaming_onchange(duthost, ptfhost, path_list, count):
     ip = duthost.mgmt_ip
     port = env.gnmi_port
     cmd = 'python /root/gnxi/gnmi_cli_py/py_gnmicli.py '
-    cmd += '--timeout 30 '
+    cmd += '--timeout 120 '
     cmd += '-t %s -p %u ' % (ip, port)
     cmd += '-xo sonic-db '
     cmd += '-rcert /root/gnmiCA.pem '