Make main_test.go more precise and clear. (#202)
Make main_test.go more precise and clear. (#202)

- Remove unused variable promPath.
- Update variable name from "prom" to "cmd", as in the Prometheus
community, "prom" means Prometheus, but we are testing
stackdriver-proemtheus-sidecar.
- Correct error messages.
Stackdriver/stackdriver-prometheus-sidecar,Stackdriver/stackdriver-prometheus-sidecar,Stackdriver/stackdriver-prometheus-sidecar