Proposal: Allow debugging `go_test` targets with Delve

I ran a Go test and wanted to debug it. I expected to find a `--go_debug` flag as an analog to the [--java_debug](https://bazel.build/reference/command-line-reference#flag--java_debug) flag:

```
bazel test my/app:test --go_debug
```

It would be great if this (or something like it) existed to run tests with Delve and wait for a debugger to be attached. It would provide a much less awkward mechanism for debugging Go code.

Start by tracing how the `go_test` target is implemented and how Bazel's `--java_debug` behavior is exposed. Determine where a Delve-backed test invocation and debugger wait would fit; done means a command such as `bazel test my/app:test --go_debug` can launch the test and wait for an attached debugger.

Written by the indexing model from the issue text.