contributing: clean up testing instructions
Fixes
## Running system tests
-Set the `GOLANG_SAMPLES_PROJECT_ID` environment variable to a suitable test project.
-
+1. Set the `GOLANG_SAMPLES_PROJECT_ID` environment variable to a suitable test project.
+1. Ensure you are logged in using `gcloud login` or set the `GOOGLE_APPLICATION_CREDENTIALS` environment variable to the path of your credentials file.
Tests are authenticated using [Application Default Credentials](https://developers.google.com/identity/protocols/application-default-credentials).
+1. Install the test dependencies:
+
+ `go get -t -d github.com/GoogleCloudPlatform/golang-samples/...`
+1. Run the tests:
-Ensure you are logged in using `gcloud login` or set the `GOOGLE_APPLICATION_CREDENTIALS` environment variable to the path of your credentials file.
+ `go test github.com/GoogleCloudPlatform/golang-samples/...`
## Contributor License Agreements
