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:

• Accepted the DCO. Commits without the DCO will delay acceptance.
• 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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            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.