Parent ticket: #101016
Related to: #109293

Summary

(Immediately after 7.15.0)

Add tests that compare the 7.15 shipped schema to the 7.16+ schema in development to ensure that the existing schema can be upgraded properly.

Ideas:

• there would be 1 test per 1 index (or in different words per rule registry client calling ruleDataService.initializeIndex())
• the tests would compare actual resulting schema (or index template) with a snapshot
• these tests could be in their own separate folder
• we could set certain CODEOWNERS for this folder

Background

The background for this is our discussions with @kobelb (see #109276 (comment) and above comments) on the "compatibility" of the current index upgrade logic with the ideas for backwards compatibility (#109293).