go: add additional environment tests for go

This PR builds out more environment tests for the Go Logging library

adds a local test environment, for testing the library in a local Docker container
added tests for json logs
added tests for additional loggers: StandardLogger, LogSync, RedirectAsJSON(os.Stdout)
updated the deployable to use reflection to find the proper test snippet, to make it more scalable
added extra test files containing tests for all go environments, and tests for all go standard out environments

In the process of writing these tests, I discovered https://github.com/googleapis/google-cloud-go/issues/6995. It seems that standard out logs add timestamp fields to the log's payload

I have already updated the golang-envtest-pin tag to point the go tests against this branch, and all tests are passing

googleapis / env-tests-logging