Possible fix for //pkg/plugin/grpc/grpc:grpc_test

[heartless-clown]

First of all I'm not very familiar with testing go code in general and the usage of github.com/bazelbuild/rules_go/go/tools/bazel_testing specifically, so I may be wrong here (also, I don't really know how to make sure changed test target works as intended). But, it seems the test code that uses bazel_testing.RunBazel() (which creates test workspace) should be called from go_bazel_test, not go_test rule.