[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

test_PortAutoNegWarm failed (1 runs remaining out of 2).
	<class 'AssertionError'>
	assert 'RESTARTCHECK...unction 267\n' == 'RESTARTCHECK succeeded\n'
    RESTARTCHECK succeeded
  - profiling:/__w/1/s/common/.libs/libswsscommon_la-consumerstatetable.gcda:Merge mismatch for function 267
	[<TracebackEntry /agent/_work/1/s/tests/test_port_an.py:262>]

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-consumerstatetable.gcda:Merge mismatch for function 267
	[<TracebackEntry /agent/_work/1/s/tests/test_port_an.py:262>]

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