Conditionally include Bazel testing library only if needed (#2279)
Conditionally include Bazel testing library only if needed (#2279)

Conditionally include the Bazel testing library. 

When using something like

```
# gazelle:map_kind go_test go_bazel_test @io_bazel_rules_go//go/tools/bazel_testing:def.bzl
```

with a directory of Bazel tests, Gazelle will automatically add this target as a
dep and duplicate entries is a build error.

bazelbuild/rules_go,bazelbuild/rules_go,bazelbuild/rules_go,bazelbuild/rules_go,bazelbuild/rules_go