[Infrastructure] Add bwc tests for rescore functionality in hybrid query

[vibrantvarun]

Add bwc tests for rescore functionality in hybrid query. PR 917

[martin-gaievski]

I've added rescorer as part of the existing rolling upgrade test in initial PR https://github.com/opensearch-project/neural-search/pull/917/files#diff-7acc444fb409f783c5ff067c7d1e5a173eac25427c04765f8af9b6f26b7e18e7. Do you mean some additional/different test in scope of this issue @vibrantvarun?

[vibrantvarun]

@martin-gaievski I think it is good. Because we already tested rescore complex functionality in integ test and unit test. For BWC one test would be fine. We can close this this issue.
cc: @minalsha