Automatic merge from submit-queue (batch tested with PRs 60324, 60269, 59771, 60314, 59941). 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>.

[GCE] Refactor ExternalLoadBalancer Tests

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

- Refactors the ExternalLoadBalancer tests to use the generated GCE mock instead of FakeCloudAddressService, FakeCloudForwardingRuleService.
- Adds hooks to populate NetworkTier on Alpha resources
- Moves shared code to top of the external loadbalancer test file
- Moves NetworkTier into a constants file at the cloud level, so it is more easily called in subpackages

**Special notes for your reviewer**:

```release-note
NONE
```