update serve-stale test to support EDE 22

update serve-stale test to support EDE 22

When EDE 3 (stale answer) was added the serve-stale tests were checking
for those exclusively, i.e. grepping for no "EDE" in the dig output when
no stale answer was expected.

However, some stale tests disable stale answers and make the
authoritative server unresponsive, effectively triggering a timed out
request thus an EDE 22. Update those tests so they still tests the
absence of EDE 3 error, but also the presence of EDE 22.

(cherry picked from commit 27f3b8950a675afe875759027a241da7c5244511)