Fix test_warning_captured_deprecated_in_pytest_6

This test started to fail in the 'py38-pytestmain' environment, the
cause being PytestRemovedIn7Warning being raised by the conftest file
of the test itself. Ignoring it is fine, the purpose of the test is to ensure
the hook is not called by pytest-xdist.

testing/acceptance_test.py

@@ -790,7 +790,7 @@ def test():
                 warnings.warn("my custom worker warning")
         """
         )
-        result = pytester.runpytest("-n1")
+        result = pytester.runpytest("-n1", "-Wignore")
         result.stdout.fnmatch_lines(["*1 passed*"])
         result.stdout.no_fnmatch_line("*this hook should not be called in this version")