Automatic merge from submit-queue (batch tested with PRs 52792, 48963). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Fix range for min value in imagepolicy admission 

**What this PR does / why we need it**:
This makes the range for imagepolicy admission work properly. Currently if we specify `1s` for the value of `AllowTTL` or `DenyTTL` in the configuration yaml, we get a message like the following: 

```
Error: failed to initialize plugins: Couldn't init admission plugin "ImagePolicyWebhook": valid value is between 1s and 30m0s, got 1s
```

This is due to a wrong comparison which is fixed in this PR. The rest of the PR just adds tests for this behaviour.