Integration test http2 improvements

[danielbeaudreau]

http2 integration test steps should store a different file than example.dcm
Storing the same file that was already stored in another step doesn't test anything because

1. StoreSCU steps don't fail if the file isn't stored
2. The CHC API does not do anything if the file is a duplicate

Also we should add a close adapter step for the http2 adapter or remove the other close adapter steps if this is not necessary.