These tests spin up two nodes of an older version of Elasticsearch,
create some stuff, shut down the nodes, start the current version,
and verify that the created stuff works.
You can run `gradle qa:full-cluster-restart:check` to run these
tests against the head of the previous branch of Elasticsearch
(5.x for master, 5.4 for 5.x, etc) or you can run
`gradle qa:full-cluster-restart:bwcTest` to run this test against
all "index compatible" versions, one after the other. For master
this is every released version in the 5.x.y version *and* the tip
of the 5.x branch.
I'd love to add more to these tests in the future but these
currently just cover the functionality of the `create_bwc_index.py`
script and start to cover the assertions in the
`OldIndexBackwardsCompatibilityIT` test.
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