[signin] ReplaceSyncPromosWithSigninPromosNewSignin in fieldtrial config - Adds the feature to testing/variations/fieldtrial_testing_config.json - fixes the tests (explicitly disable the feature when needed) Bug: None Change-Id: I1f18cac1bd22c733d224765e0ed24f36a76868e7 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7693776 Reviewed-by: Ankush Singh <[email protected]> Reviewed-by: Colin Blundell <[email protected]> Commit-Queue: David Roger <[email protected]> Cr-Commit-Position: refs/heads/main@{#1606049} NOKEYCHECK=True GitOrigin-RevId: 38071c38d15ddd3ffc5573623d9c8564554fb4d1