[vstest] Only collect stdout of orchagent_restart_check in vstest

[bingwang-ms]

What I did
This PR is to fix the flaky test cases in vstest.
We frequently saw test failures in PR testing like below

def test_warm_restart_all_interfaces(self,dvs,testlog):
        self.setup_db(dvs)
    
        tbl = swsscommon.Table(self.cdb,"PORT")
        for key in tbl.getKeys():
            self.add_storm_control_on_interface(dvs,key,"broadcast",1000000)
            self.add_storm_control_on_interface(dvs,key,"unknown-unicast",2000000)
            self.add_storm_control_on_interface(dvs,key,"unknown-multicast",3000000)
        #dvs.runcmd("config save -y")
        # enable warm restart
        dvs.warm_restart_swss("true")
    
        # freeze orchagent for warm restart
        (exitcode, result) = dvs.runcmd("/usr/bin/orchagent_restart_check")
>       assert result == "RESTARTCHECK succeeded\n"
E       AssertionError: assert 'RESTARTCHECK...unction 503\n' == 'RESTARTCHECK succeeded\n'
E         + RESTARTCHECK succeeded
E         - RESTARTCHECK failed
E         - profiling:/__w/1/s/common/.libs/libswsscommon_la-countertable.gcda:Merge mismatch for function 503

The output is produced by gcov. However, it's collected into the output of orchagent_restart_check and leads to test failure because the output is not as expected.

- profiling:/__w/1/s/common/.libs/libswsscommon_la-countertable.gcda:Merge mismatch for function 503

This PR fixes the issue by adding an argument include_stderr to runcmd. When include_stderr is set to False, only stdout is collected.

Why I did it
This PR is to fix the flaky test cases in vstest.

How I verified it
Verified by running vstest locally.

Details if related