Need a darwin kokoro runner to run bazel test

[stanley-cheung]

This comes from #7538

In that PR, we need to fix the bazel target :protobuf_objc, but we can't add the test target to kokoro/linux/bazel/build.sh because it's a linux kokoro runner. The :protobuf_objc target probably have something in it that says that will only build in darwin.

BUILD:47:1: in cc_toolchain_suite rule @local_config_cc//:toolchain: cc_toolchain_suite '@local_config_cc//:toolchain' does not contain a toolchain for cpu 'darwin_x86_64'                     
ERROR: Analysis of target '//:protobuf_objc' failed; build aborted: Analysis of target '@local_config_cc//:toolchain' failed; build aborted   

So we need a darwin kokoro runner to run this bazel test.

[haberman]

Closing as obsolete.