pytest-xdist hides warnings from pytest

Consider the following example:
```
import pytest

@pytest.fixture
def foo():
    return 42

def test_foo():
    foo()
```
As of pytest 3.8, running this test gives the expected warning ("Fixtures are not meant to be called directly, etc.").  But running e.g. `pytest -n2` (pytest-xdist 1.23.0) causes no warnings to be shown.