test_data includes a file only used for ES 7 and up

[jameslamb]

See this comment from #161 for background.

File test_data/sample_es7.json was introduced in that PR to handle the fact that _type in ES7 was removed (i.e. indices can no longer contain documents with multiple types). This isn't a big problem, but it would be cool if we could figure out how to eliminate this added complexity in the CI and remove that file, possibly by adding some argument to the ES7 mapping that says the default document type is line.