Explicitly set junit_family=xunit1 @ pytest config

This change hides the deprecation warning and protects from
failures in pytest==6.0.0

Ref:
* https://github.com/ansible/ansible/pull/66445#discussion_r372530176

Co-Authored-By: Matt Clay <matt@mystile.com>

test/lib/ansible_test/_data/pytest.ini

@@ -1,3 +1,9 @@
 [pytest]
 xfail_strict = true
 mock_use_standalone_module = true
+# It was decided to stick with "legacy" (aka "xunit1") for now.
+# Currently used pytest versions all support xunit2 format too.
+# Except the one used under Python 2.6 — it doesn't process this option
+# at all. Ref:
+# https://github.com/ansible/ansible/pull/66445#discussion_r372530176
+junit_family = xunit1