Automatic merge from submit-queue (batch tested with PRs 60499, 61715, 61688, 61300, 58787). 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>.
Unit tests for external load balancer
**What this PR does / why we need it**:
Unit test for external load balancer. Increase the code coverage of gce_loadbalancer_external.go from 61.6% to 76.5%.
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**Release note**:
```release-note
NONE
```
