Override EE migration folders for `require_migration!`

!45886 (merged) added EE-specific file locations into FOSS codebase, instead of defining ee-only directories (!45886 (diffs)) directly in spec/support/helpers/require_migration.rb it would be cleaner to override migration_folders method in prepended EE module.

The following discussion from !45886 (merged) should be addressed:

• @jprovaznik started a discussion: (+4 comments)

  Thanks @minac, LGTM, just one minor question.