Avoid creating unnecessary records in spec/finders/merge_requests_finder_spec.rb

The following discussion from !18119 (merged) should be addressed:

• @rspeicher started a discussion:

  I think we should have avoided creating a bunch of additional MRs for every single test, especially when those tests aren't concerned with filtering by WIP status. We should add a context "when filtering by WIP status" block and scope these additional objects there. This would have made the changes on lines 38, 39, 46, 52, 60, 68, and 74 unnecessary. Will open a ~"technical debt" issue to resolve.