Bazel go-tutorial missing tests and race tests

We love Go because of its easy-to-use concurrency and therefore race tests are critical for anyone wanting to use Bazel with Go.

The current go-tutorial has neither examples of tests, nor race tests.  This is a major deficiency in the tutorial.

https://bazel.build/start/go
https://github.com/bazelbuild/examples/tree/main/go-tutorial/stage3

