fix: use Contains or ErrorContains with testify

[mmorel-35]

Thank you for contributing to Velero!

Please add a summary of your change

use Contains or ErrorContains with testify

Does your change fix a particular issue?

It uses (assert|require).Contains(t,...) or (assert|require).ErrorContains(t, err,...) when possible It also removes (assert|require).Error(t, err,...) as it is redundant with (assert|require).ErrorContains(t, err,...) behavior

Please indicate you've done the following:

• [x] Accepted the DCO. Commits without the DCO will delay acceptance.
• [x] Created a changelog file (make new-changelog) or comment /kind changelog-not-required on this PR.
• [ ] Updated the corresponding documentation in site/content/docs/main.

[mmorel-35]

/kind changelog-not-required

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 58.99%. Comparing base (732b87b) to head (d06601e). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #8322 +/- ## ========================================== - Coverage 59.17% 58.99% -0.19% ========================================== Files 367 367 Lines 30849 38847 +7998 ========================================== + Hits 18256 22918 +4662 - Misses 11130 14467 +3337 + Partials 1463 1462 -1 ```

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[kaovilai]

Please close and reopen PRs

8066

[anshulahuja98]

Hopefully this is okay to merge before 1.15 RC.