Automatic merge from submit-queue (batch tested with PRs 54030, 54041). 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 admission webhook test failure in go1.9

**What this PR does / why we need it**:

Go 1.9 has some better checks for SAN extensions, notably:
https://github.com/golang/go/commit/630e93ed2d8a13226903451a0e85e62efd78cdcd

We recently added ServerName to the rest client TLSConfig:
https://github.com/dims/kubernetes/commit/186a0684d582272aa2fbfec390aea5fbb88a8bc6#diff-eb3573c6b11347dc3793c477726480efR262

So we need to be sure that the ServerName "webhook-test.default.svc" is
present in the certificates that we generate."

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #

Fixes #54020

**Special notes for your reviewer**:

**Release note**:

```release-note
NONE
```