Fix 1.14 nightly issues

[danfengliu]

1. Add sleep for native snapshot tests when using test.go interface;
2. Add --confirm for velero plugin add CLI as new feature introduced.

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[codecov[bot]]

Codecov Report

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

Project coverage is 58.60%. Comparing base (5f9c53a) to head (8a3f2f4).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #7727 +/- ## ======================================= Coverage 58.60% 58.60% ======================================= Files 343 343 Lines 28593 28593 ======================================= Hits 16757 16757 Misses 10418 10418 Partials 1418 1418 ```

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