run this before and after installing pytest-xdist in the environment

conftest.py

def pytest_terminal_summary(terminalreporter):
    config = terminalreporter.config
    if not test_cases or config.option.verbose < 1:
        return
    tw = terminalreporter._tw
    tw.sep('-', 'this line will not show up with -n 1')
    tw.line('this line should show up')

test_it.py

pytest_plugins = 'pytester'

def test_terminal_summary_with_xdist_dash_n(testdir):
    try:
        result = testdir.runpytest('--version')
        result.stderr.fnmatch_lines(['*pytest-xdist*'])
    except Exception:
        import pytest
        pytest.skip("this test requires pytest-xdist")

    testdir.makepyfile("""
        import pytest
        def test_whatever():
            pass
        """)
    result = testdir.runpytest('--verbose', '-n 1')
    result.stdout.fnmatch_lines(['*this line will not show up with -n 1*',
                                 '*this line should show up*'])