Use modernized versions of CreateTestFormField in CreditCardSaveManager.

This CL is a mechanical change and does not affect functionality - it
simply replaces out parameters by return values.

Bug: 1465839
Change-Id: I44ab50c9e8d73961bfe9ea75d56d6dab75b29e57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4783474
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Auto-Submit: Jan Keitel <jkeitel@google.com>
Reviewed-by: Christoph Schwering <schwering@google.com>
Commit-Queue: Jan Keitel <jkeitel@google.com>
Cr-Commit-Position: refs/heads/main@{#1184553}