test_as_errors: use subprocess with `-W`

Ref: https://github.com/pytest-dev/pytest/pull/4981

testing/test_warnings.py

@@ -98,7 +98,9 @@ def test_as_errors(testdir, pyfile_with_warnings, method):
             filterwarnings=error
             """
         )
-    result = testdir.runpytest(*args)
+    # Use a subprocess, since changing logging level affects other threads
+    # (xdist).
+    result = testdir.runpytest_subprocess(*args)
     result.stdout.fnmatch_lines(
         [
             "E       UserWarning: user warning",