Fix flaky test.

test_DELETE_x_container_headers_with_more_container_replicas() would
sometimes fail, and it's because a test helper method was defaulting
to sorting the captured headers by X-Container-Partition. Well, in
tests we use FakeRing, and the partition is always 1, so that sorting
didn't buy us much. Now the sorting is done by X-Container-Device,
which actually differs meaningfully.

Change-Id: Ibe5b2fcd3f23280ed2caaa703111a98861331866