Consolidate spec/support for cross_database_modification setup

Dylan Griffith (ex GitLab)requested to merge
consolidate-cross-db-modification-spec-support into master

What does this MR do and why?

These 2 files are configuring things for cross database modification stuff in RSpec and the files have very similar names which makes it easy to get them mixed up. Considering they are for the same thing it makes sense to put them in the same file.

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