test(mysql): update system tests to use Cloud SQL proxy
test(mysql): update system tests to use Cloud SQL proxy

```
./cloud_sql_proxy -instances=pso-kokoro-resources:us-central1:data-validator-mysql=tcp:3306
```

Change-Id: I22110ed6696a60157a883e0d55d2f71dd872c56f

GoogleCloudPlatform/professional-services-data-validator,GoogleCloudPlatform/professional-services-data-validator