Add Rails/SaveBang Rubocop rule for tests

Description of the proposal

When discussing to add notice about usage of "update!" instead of "update", we realized that it is better to use a Rubocop rule for this.

This MR adds Rails/SaveBang rule for tests.

Check-list

• Make sure this MR enables a static analysis check rule for new usage but ignores current offenses
• Mention this proposal in the relevant Slack channels (e.g. #development, #backend, #frontend)
• The MR doesn't have significant objections, and is getting a majority of 👍 vs 👎 (remember that we don't need to reach a consensus)
• Create a follow-up issue to fix the current offenses as a separate iteration: #220040 (closed)
• Follow the review process as usual
• Once approved and merged by a maintainer, mention it again:
  • In the relevant Slack channels (e.g. #development, #backend, #frontend)
  • (Optional depending on the impact of the change) In the Engineering Week in Review

/cc @gitlab-org/maintainers/rails-backend