Automatic merge from submit-queue (batch tested with PRs 49871, 49422, 49092, 49858, 48999)
Add KUBE_COVER help to "make test"
**What this PR does / why we need it**:
Add help info on `KUBE_COVER` to tell contributors how to run test with code coverage.
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #
**Special notes for your reviewer**:
/sig contributor-experience
**Release note**:
```release-note
NONE
```
5d4d422be2