ElasticV7QueryChangesTest: Close indices after test
For each test method, new indices are created with a unique name based
on the test method name. This results in 3 indices (i.e. one each for
accounts, changes and groups) for each test method that is run, which
in turn results in the number of allocated shards increasing.
In Elasticsearch 7.0 a shard limit is introduced  which so far only
causes a warning. However in a future version (not clear if this means
the final 7.0 release or a subsequent one) the limit will be enforced
and result in an error.
After each test, close the indices that were created. This results in
the shards being deallocated, and prevents exceeding the limit.
Bug: Issue 10120