upgrade/upgrades: allow CreatedAtNanos to be set when validating migration #87462
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Schema change upgrade migrations to system tables are made idempotent by checking that the descriptor reaches some expected state. In order to ensure that it is in that expected state, some volatile fields need to be masked. We forgot to mask CreatedAtNanos. We also lost the testing which came with these helper functions we use.
The vast majority of this PR is reviving testing from #66889.
Fixes #85228.
Release justification: Import bug fix for backport
Release note (bug fix): Some upgrade migrations perform schema changes on system tables. Those upgrades which added indexes could, in some cases, get caught retrying because they failed to detect that the migration had already occurred due to the existence of a populated field. When that happens, the finalization of the new version can hang indefinitely and require manual intervention. This bug has been fixed.