🐞 V3: less flaky unit tests

The test failed because we had an extra event in the database. The extra
event was an artifact of a separate test which left a lingering usage
event in the database. We now delete the lingering event at test
completion.

Fixes:
<https://capi.ci.cf-app.com/teams/main/pipelines/capi/jobs/cc-unit-tests/builds/2552>
```
1) App Usage Events GET /v3/app_usage_events behaves like permissions for list endpoint as an admin_read_only returns the correct response status and resources
     Failure/Error: expect({ resources: parsed_response['resources'] }).to match_json_response({ resources: expected_response_objects })

       Summary:
             + resources[0]: {:guid=>"3e668ba8-a72e-4a66-beab-c43629712a15", :created_at=>"2020-06-09T11:32:34Z", :updated_at=>"2020-06-09T11:32:34Z", :state=>{:current=>"STARTED", :previous=>"STARTED"}, :app=>{:guid=>"602f702c-5f78-4249-8d38-8f9386f3fca1", :name=>"name-20104"}, :process=>{:guid=>"3e668ba8-a72e-4a66-beab-c43629712a15", :type=>nil}, :space=>{:guid=>"fc22f581-4a7e-461b-aeb4-885614139c96", :name=>"name-20101"}, :organization=>{:guid=>"5894354a-53f5-4483-bc90-b0e08cb86beb"}, :buildpack=>{:guid=>nil, :name=>nil}, :task=>{:guid=>nil, :name=>nil}, :memory_in_mb_per_instance=>{:current=>1024, :previous=>1024}, :instance_count=>{:current=>1, :previous=>1}}
     Shared Example Group: "permissions for list endpoint" called from ./spec/request/app_usage_events_spec.rb:84
```

[#173251275]

Co-authored-by: Brian Cunnie <bcunnie@pivotal.io>
Co-authored-by: Teal Stannard <tstannard@pivotal.io>

Author: Brian Cunnie

spec/request/app_usage_events_spec.rb

@@ -135,6 +135,10 @@
       h.freeze
     end
 
+    after 'clean-up to avoid test pollution' do
+      VCAP::CloudController::AppUsageEvent.all.first.delete
+    end
+
     it_behaves_like 'permissions for single object endpoint', ALL_PERMISSIONS do
       let(:after_request_check) do
         lambda do